Voice Over LTE (VoLTE), SIP and the IMS

Did you know that SK Telecom launched the world’s first commercial VoLTE service in 2012, amassing 4.5 million users by mid-2013 and cutting call setup times to under 2.5 seconds, while operators like Telefónica Deutschland now route over 80% of calls via VoLTE and expert teams optimize thousands of sites to win performance challenges for major carriers?

Course Overview

The Voice Over LTE (VoLTE), SIP and the IMS course by Rcademy is designed to equip radio and packet core engineers, IP network engineers, protocol specialists, technical managers, project managers, product managers, network optimization and performance engineers, system engineers, technical marketing and strategy professionals, students, researchers, and consultants with comprehensive understanding of VoLTE architecture, IMS core network functions, and end-to-end voice service delivery over LTE/EPS networks. Participants gain expert knowledge of Session Initiation Protocol (SIP), Session Description Protocol (SDP), Diameter protocols for VoLTE signaling, IMS architecture including Call Session Control Functions (CSCF), HSS, Application Servers, and Policy and Charging Control (PCC) framework.​

Without specialized VoLTE and IMS training, professionals may struggle to decode IMS registration procedures, analyze SIP call flows and SDP offer/answer exchanges, configure Policy and Charging Rules Function (PCRF) for QCI 1 GBR bearers, implement SRVCC handover signaling, or optimize VoLTE networks through RAN planning and codec configuration, limiting their ability to support commercial VoLTE deployments and maintain superior voice quality. This comprehensive course provides a structured path to mastery across VoLTE evolution and market drivers, LTE/EPS network architecture, IMS architecture and identities, SIP and Diameter protocols, VoLTE registration and call flows, PCC framework, supplementary services, interworking with legacy networks, SRVCC and VoWiFi integration, and systematic troubleshooting and optimization, preparing attendees to lead VoLTE deployment and quality assurance initiatives.​

The Voice Over LTE (VoLTE), SIP and the IMS course covers evolution to VoLTE and market drivers, LTE/EPS network architecture for VoLTE, IP Multimedia Subsystem (IMS) architecture, IMS identities and addressing, Session Initiation Protocol (SIP) fundamentals, Session Description Protocol (SDP), Diameter protocol in IMS, RTP/RTCP and media transmission, VoLTE registration procedures, VoLTE call establishment and termination, Policy and Charging Control (PCC) for VoLTE, VoLTE supplementary services (MMTel), VoLTE interworking with legacy networks, Circuit-Switched Fallback (CSFB), Single Radio Voice Call Continuity (SRVCC), Voice over WiFi (VoWiFi) architecture, VoWiFi call flows and mobility, SMS over IMS and messaging, VoLTE planning and optimization, VoLTE KPIs and Quality of Experience (QoE), VoLTE troubleshooting and signaling analysis, and practical labs with real-world case studies. Participants learn to decode SIP methods (REGISTER, INVITE, ACK, BYE, CANCEL, UPDATE, PRACK), analyze Diameter applications (Cx/Dx, Sh/Dh, Rx), configure QoS Class Identifier (QCI) for VoLTE, perform SRVCC handover trace analysis, optimize codecs (AMR, AMR-WB, EVS), and use protocol analyzers like Wireshark for systematic troubleshooting.​

Real-world cases show how SK Telecom launched the world’s first commercial VoLTE service under the “HD Voice” brand in August 2012, achieving over 4.5 million active VoLTE users by May 2013 with dramatically improved audio quality through high-quality codecs handling 2.2 times wider frequency bandwidth than 3G voice and call setup time reduced to 0.25-2.5 seconds.

Studies also show that Telefónica Deutschland became the first network operator in Germany to provide VoLTE and VoWiFi for all customers (postpaid, prepaid, home brands, and partner brands) by March 2018, with 80% of all telephone calls on its network routed via VoLTE by 2024, delivering HD voice quality and significantly shorter call setup times.

Take charge of your VoLTE expertise. Enroll now in the Rcademy Voice Over LTE (VoLTE), SIP and the IMS course to master the IMS architecture and signaling skills that drive high-quality voice services.

Who Should Attend?

The Voice Over LTE (VoLTE), SIP and the IMS course by Rcademy is ideal for:

• Radio and packet core engineers
• IP network engineers and protocol specialists
• Technical managers and project managers
• Product managers overseeing VoLTE projects
• Network optimization and performance engineers
• System engineers performing VoLTE testing
• Integration and troubleshooting specialists
• Technical marketing and strategy professionals
• VoLTE deployment engineers
• IMS core network engineers
• SIP signaling specialists
• QoS and policy control engineers
• VoWiFi implementation engineers
• Students and researchers in 4G voice technologies
• Consultants expanding VoLTE knowledge

What are the Training Goals?

The main objectives of the Voice Over LTE (VoLTE), SIP and the IMS course are to enable professionals to:

• Master comprehensive VoLTE architecture, IMS core network functions, and end-to-end voice service delivery over LTE/EPS networks.
• Develop expert-level knowledge of Session Initiation Protocol (SIP), Session Description Protocol (SDP), and Diameter protocols for VoLTE signaling.
• Understand IMS architecture, Call Session Control Functions (CSCF), HSS, Application Servers, and their roles in multimedia service delivery.
• Analyze detailed VoLTE call flows including registration, session establishment, termination, and supplementary services using trace-level signaling.
• Apply Policy and Charging Control (PCC) framework, QoS mechanisms, and bearer management for optimal VoLTE quality of experience.
• Design and troubleshoot VoLTE interworking scenarios including CS Fallback (CSFB), SRVCC, and interconnection with PSTN/legacy networks.
• Implement Voice over WiFi (VoWiFi) solutions, understand trusted/untrusted access, and optimize handover between VoLTE and VoWiFi.
• Optimize VoLTE networks through RAN planning, codec configuration, DRX settings, and end-to-end KPI monitoring for superior voice quality.

How Will This Training Course Be Presented?

At Rcademy, the extensive focus is laid on the relevance of the training content to the audience. Thus, content is reviewed and customised as per the professional backgrounds of the audience.

The training framework includes:

• Expert-led lectures by senior VoLTE and IMS professionals using audio-visual sessions
• Hands-on exercises with SIP/SDP message decoding and Diameter signaling analysis
• Interactive workshops for VoLTE call setup trace analysis and dedicated bearer establishment
• Case studies covering SK Telecom world-first VoLTE deployment, Telefónica Deutschland nationwide rollout, and Ericsson VoLTE RAN optimization
• Practical labs for SRVCC handover analysis, VoWiFi session establishment, and systematic troubleshooting

The theoretical part of training is delivered by an experienced professional from the relevant domain, using audio-visual presentations. This VoLTE-focused approach ensures professionals translate theory into practical workflows through IMS registration analysis, call flow decoding, PCC policy configuration, QoS optimization, and systematic troubleshooting methodologies.​

This comprehensive certification model ensures participants gain both VoLTE fundamentals and hands-on proficiency to immediately apply IMS architecture and signaling expertise in VoLTE deployment, optimization, testing, and troubleshooting roles.​

Register now to experience a rigorous, hands-on learning journey designed to equip you for leading VoLTE deployment, IMS integration, and network optimization projects.

Course Syllabus

Module 1: Evolution to VoLTE and Market Drivers

• Voice service evolution from 2G/3G circuit-switched to 4G packet-switched VoLTE.​
• Market drivers and business case for VoLTE deployment by operators.​
• GSMA PRD IR.92 specifications and VoLTE service requirements.​
• Alternative voice solutions: CS Fallback (CSFB), VoLGA, and over-the-top (OTT) services comparison.​
• VoLTE benefits: HD voice quality, faster call setup, spectrum efficiency, video calling.​

Module 2: LTE/EPS Network Architecture for VoLTE

• LTE system architecture: E-UTRAN and Evolved Packet Core (EPC) overview.​
• EPC components: MME, S-GW, P-GW, HSS, PCRF and their roles in VoLTE.​
• EPS bearer model: default bearer and dedicated bearer concepts for voice.​
• LTE attach procedure and PDN connectivity for IMS access.​
• Radio functionality and resource allocation for voice traffic.​

Module 3: IP Multimedia Subsystem (IMS) Architecture

• IMS architecture overview and service delivery framework.​
• Call Session Control Functions: Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), Serving-CSCF (S-CSCF).​
• Home Subscriber Server (HSS) for subscriber data and authentication.​
• Application Servers (AS): SIP AS, telephony application server (TAS), and service logic execution.​
• Media Resource Function (MRF): MRFC and MRFP for conferencing and announcements.​
• IMS reference points and interfaces: Gm, Mw, ISC, Cx/Dx, Mg, Mr.​

Module 4: IMS Identities and Addressing

• IMS Public User Identity (IMPU): SIP URI and Tel URI formats.​
• IMS Private User Identity (IMPI) for authentication and subscription.​
• Relationship between IMPU, IMPI, and device identities (IMEI, MSISDN).​
• Public Service Identity (PSI) for application and service addressing.​
• Identity management and subscriber provisioning in HSS.​

Module 5: Session Initiation Protocol (SIP) Fundamentals

• SIP architecture: user agents, proxy servers, registrars, back-to-back user agents (B2BUA).​
• SIP request methods: REGISTER, INVITE, ACK, BYE, CANCEL, UPDATE, PRACK, INFO.​
• SIP response codes: provisional (1xx), success (2xx), redirection (3xx), client error (4xx), server error (5xx).​
• SIP message structure: request/status line, headers, and message body.​
• SIP transactions, dialogs, and session lifecycle.​
• SIP addressing and routing mechanisms.​

Module 6: Session Description Protocol (SDP)

• SDP purpose and role in multimedia session negotiation.​
• SDP message structure: session-level and media-level descriptions.​
• SDP offer/answer model in SIP-based call establishment.​
• Media types, codecs, transport protocols, and attributes in SDP.​
• Bandwidth negotiation and quality parameters configuration.​

Module 7: Diameter Protocol in IMS

• Diameter protocol architecture and role in IMS for AAA and policy.​
• Diameter message format: commands, AVPs (Attribute-Value Pairs), sessions.​
• Diameter applications: Cx/Dx (I-CSCF/S-CSCF to HSS), Sh/Dh (AS to HSS), Rx (P-CSCF to PCRF).​
• Authentication and subscriber data retrieval via Diameter.​
• Diameter routing and transport over SCTP.​

Module 8: RTP, RTCP, and Media Transmission

• Real-time Transport Protocol (RTP) for voice media transport.​
• RTP header structure, payload types, and sequence numbering.​
• RTCP (RTP Control Protocol) for feedback and quality reporting.​
• Voice codecs for VoLTE: AMR, AMR-WB (Wideband), EVS (Enhanced Voice Services).​
• Robust Header Compression (RoHC) for bandwidth efficiency.​

Module 9: VoLTE Registration Procedures

• LTE attach and default EPS bearer establishment for IMS connectivity.​
• PDN connection setup to IMS domain (APN: IMS).​
• IMS registration: SIP REGISTER message flow from UE to IMS core.​
• IMS authentication using IMS-AKA and integration with LTE authentication.​
• P-CSCF discovery: DHCPv6, PCO (Protocol Configuration Options).​
• Registration state management and re-registration procedures.​

Module 10: VoLTE Call Establishment and Termination

• Mobile-originated VoLTE call: SIP INVITE flow through IMS core.​
• Mobile-terminated VoLTE call: paging, SIP INVITE delivery, alerting.​
• SDP offer/answer exchange and codec negotiation during call setup.​
• Dedicated bearer establishment for voice media: QoS negotiation and bearer activation.​
• 180 Ringing, 200 OK, and ACK message flows.​
• Call termination: BYE request and session teardown procedures.​

Module 11: Policy and Charging Control (PCC) for VoLTE

• PCC architecture: PCRF, PCEF (P-GW), BBERF, and their interactions.​
• Rx interface: P-CSCF to PCRF signaling for VoLTE session policies.​
• Gx interface: PCRF to PCEF for bearer establishment and QoS enforcement.​
• QoS Class Identifier (QCI) for VoLTE: QCI 1 for conversational voice.​
• Guaranteed Bit Rate (GBR) bearers and resource reservation.​
• Dynamic policy rules and service data flow detection.​

Module 12: VoLTE Supplementary Services (MMTel)

• Multimedia Telephony Service (MMTel) in IMS Application Servers.​
• Call forwarding (unconditional, busy, no reply, not reachable).​
• Call waiting and call hold services.​
• Three-way conferencing and multiparty calling.​
• Call transfer (blind, consultative) procedures.​
• Calling Line Identification Presentation/Restriction (CLIP/CLIR).​

Module 13: VoLTE Interworking with Legacy Networks

• VoLTE to PSTN call flows: MGCF and MGW roles.​
• VoLTE to 3G CS (Circuit-Switched) call interworking.​
• Breakout Gateway Control Function (BGCF) for routing decisions.​
• ENUM and DNS lookup for number translation.​
• Incoming calls from CS domain to VoLTE subscribers.​

Module 14: Circuit-Switched Fallback (CSFB)

• CSFB principles and architecture for voice service continuity.​
• CSFB call flow: combined attach, service request, and fallback to 3G/2G.​
• SGs interface between MME and MSC for CSFB signaling.​
• CSFB to GERAN and UTRAN procedures.​
• CSFB call setup delays and optimization strategies.​

Module 15: Single Radio Voice Call Continuity (SRVCC)

• SRVCC principles and handover from VoLTE to 3G/2G CS domain.​
• SRVCC architecture: MSC Server enhanced for SRVCC, STN-SR.​
• Basic SRVCC (bSRVCC), enhanced SRVCC (eSRVCC), and evolved variants.​
• SRVCC handover signaling flows and session continuity.​
• Reverse SRVCC (rSRVCC) from CS to VoLTE.​
• Video SRVCC (vSRVCC) for video call continuity.​

Module 16: Voice over WiFi (VoWiFi) Architecture

• VoWiFi standardization and integration with IMS.​
• Trusted and untrusted non-3GPP access to EPC.​
• Evolved Packet Data Gateway (ePDG) for secure WiFi connectivity.​
• IKEv2 and IPsec tunnel establishment for untrusted WiFi access.​
• ANDSF (Access Network Discovery and Selection Function) role.​
• Hotspot 2.0 (Passpoint) for seamless WiFi authentication.​

Module 17: VoWiFi Call Flows and Mobility

• VoWiFi registration and IMS PDN connection procedures.​
• VoWiFi call establishment: SIP signaling over IPsec tunnel.​
• VoWiFi to VoLTE handover and vice versa.​
• Diameter signaling (S2a, S2b) for VoWiFi authentication and policy.​
• Media streams and QoS for VoWiFi calls.​

Module 18: SMS Over IMS and Messaging

• SMS over IP architecture and IMS integration.​
• IP-SM-GW (IP Short Message Gateway) for SMS routing.​
• SMS over SGs for CSFB-enabled devices.​
• Rich Communication Services (RCS) and advanced messaging over IMS.​

Module 19: VoLTE Planning and Optimization

• VoLTE coverage planning: RSRP, RSRQ, and SINR requirements.​
• VoLTE capacity planning and traffic modeling.​
• Radio resource allocation and semi-persistent scheduling (SPS) for VoIP.​
• DRX (Discontinuous Reception) configuration for battery life vs. latency trade-offs.​
• Codec optimization: AMR vs. AMR-WB vs. EVS selection.​
• Mobile backhaul and transmission requirements for VoLTE.​

Module 20: VoLTE KPIs and Quality of Experience (QoE)

• VoLTE Key Performance Indicators: call setup success rate, call drop rate, call setup time.​
• Mean Opinion Score (MOS) for VoLTE voice quality assessment.​
• Audio Quality KPIs (AQ-KPIs): jitter, packet loss, delay.​
• End-to-end QoE monitoring and optimization across RAN, EPC, IMS, and transport.​
• UE feature group indicators (FGI) and VoLTE capability validation.​

Module 21: VoLTE Troubleshooting and Signaling Analysis

• Protocol analyzer tools for VoLTE: Wireshark, NetScout, Acme Packet, EXFO.​
• Capturing and analyzing VoLTE call flows: registration, call setup, bearer establishment.​
• SIP message decoding and common errors troubleshooting.​
• Diameter message analysis on Cx, Sh, and Rx interfaces.​
• Common VoLTE issues: registration failures, call setup failures, one-way audio, call drops.​
• Root cause analysis methodologies and trace correlation techniques.​

Module 22: Practical Labs and Case Studies

• Hands-on: LTE attach and IMS registration trace analysis.​
• Hands-on: VoLTE call setup signaling analysis with SIP/SDP decoding.​
• Hands-on: Dedicated bearer establishment and QoS policy enforcement analysis.​
• Hands-on: SRVCC handover trace analysis and session continuity validation.​
• Hands-on: VoWiFi session establishment and handover to VoLTE analysis.​
• Case studies: commercial VoLTE deployments, optimization challenges, and best practices.

Training Impact

The impact of Voice Over LTE (VoLTE), SIP and the IMS training is visible in how operators pioneer world-first VoLTE deployments achieving millions of subscribers, enable nationwide VoLTE/VoWiFi services migrating majority voice traffic to LTE, and deliver measurable KPI improvements through systematic RAN optimization.​

SK Telecom (Korea) – World’s First VoLTE Deployment Achieving 4.5 Million Users and Pioneering SRVCC

Implementation: SK Telecom, South Korea’s leading mobile operator, launched the world’s first commercial VoLTE service under the “HD Voice” brand in August 2012, followed by LG U+’s “Jieum” service in the same month and KT’s “HD Voice” two months later, making Korea the first nation to fully commercialize inter-operator VoLTE service across all three major operators. Dr. J.W. Byun, CTO of SK Telecom, revealed at the LTE World Summit in Amsterdam (June 2013) that the operator had achieved over 4.5 million active VoLTE users by the end of May 2013, demonstrating dramatic improvements in audio quality through high-quality codecs capable of handling 2.2 times wider frequency bandwidth than 3G voice and reducing call setup time from 5 seconds on 3G to just 0.25-2.5 seconds on LTE. SKT launched VoLTE as it sought to cut ties with legacy voice networks as well as better compete with over-the-top VoIP services, with Byun reiterating that VoLTE dramatically improves audio quality relying on high quality audio codecs. In 2013, SK Telecom became the first operator globally to launch Single Radio Voice Call Continuity (SRVCC) service, complementing VoLTE by supporting seamless handovers between 3G and LTE networks to achieve uninterrupted voice services even in congested networks or at the edge of LTE coverage.​

Results: All Korean operators saw increased ARPU after introducing LTE and VoLTE services, with LG U+ experiencing particularly strong gains, validating the business case for VoLTE deployment. Byun acknowledged that with the same network equipment available to all carriers, it was increasingly difficult to find ways of differentiating offerings, yet SK Telecom successfully defended its dominant market position through early VoLTE adoption and value-added services planning. What occupies his time at the moment is developing a “dynamic scheduler” which gives SKT better traffic management capabilities, the CTO said, with data, voice and multimedia all travelling over LTE networks in such large quantities making management of the traffic now of the utmost importance. This pioneering deployment required comprehensive expertise in IMS architecture, SIP/SDP signaling, LTE bearer management, SRVCC handover procedures, codec optimization, and end-to-end QoE monitoring, exactly the advanced skillset developed through professional VoLTE, SIP, and IMS certification training.

Telefónica Deutschland (Germany) – First Nationwide VoLTE/VoWiFi for All Customers with 80% Voice Traffic Migration

Implementation: Telefónica Deutschland became the first network operator in Germany to provide Voice over LTE (VoLTE) and Voice over WiFi (VoWiFi) for all customers of its home brands (O2) and partner brands, including both postpaid and prepaid segments, completing the rollout by March 2018. Cayetano Carbajo Martín, Chief Technology Officer, stated: “We set new standards for our customers’ mobile freedom. This is why, as the first network operator in Germany, we will offer WiFi calls and Voice over LTE via our high-performance LTE network for all our customers”. VoLTE implementation enabled optimal voice connections via the LTE (4G) network with HD-voice quality and significantly shorter call setup times compared to legacy circuit-switched voice, requiring only a VoLTE-enabled mobile device with no additional provider activation needed, with customers simply adjusting settings once manufacturers provided VoLTE-enabled firmware. VoWiFi extended service quality by allowing customers to make calls in best speech quality in places with WiFi access even when they had weak or no mobile network reception, with all important features like call forwarding and voicemail remaining available and seamless handover between WiFi and VoLTE in the 4G network without call interruption.​

Results: By 2024, Telefónica Germany reported that 80 percent of all telephone calls on its network were made via VoLTE with the percentage continuing to rise, with customers benefiting from faster call connection rates, better voice quality, and the ability to use the internet in parallel while on calls. The operator noted that voice calls over the mobile network are more popular than ever despite the growth of messaging services, with customers making a total of ten billion minutes of calls per month, and in 2023, its network witnessed over 124 million minutes of calls, which is 25 percent more than five years ago. Telefónica Germany stated “We are witnessing a resurgence of voice telephony. During the corona period, the volume of voice calls has increased significantly and since then, it has not returned to the level before”. This large-scale deployment required extensive knowledge of IMS registration procedures, SIP call flows, VoWiFi architecture (ePDG, IKEv2/IPsec for untrusted WiFi access), policy and charging control (Rx/Gx interfaces), seamless VoWiFi-to-VoLTE handover procedures, and end-to-end quality optimization, directly reflecting the comprehensive VoLTE, SIP, and IMS expertise taught in professional certification courses.​

AT&T, Verizon, and SingTel – Large-Scale VoLTE RAN Optimization Delivering Measurable KPI Improvements

Implementation: Ericsson’s VoLTE RAN optimization team executed major deployment and optimization campaigns for AT&T (over 8,000 sites optimized) and SingTel (1,700+ sites optimized), applying systematic RAN parameter tuning and advanced troubleshooting methodologies to achieve measurable improvements in voice quality and network KPIs. The optimization workflow included SRVCC border optimization to ensure seamless handover from VoLTE to 3G/2G circuit-switched networks at LTE coverage edges, mobile-terminated RRC re-establishment feature optimization to reduce call drops when devices transition between radio states, feature workarounds to address vendor-specific issues affecting call setup and retention, downlink inter-cell interference coordination (DL ICIC) feature testing to mitigate interference in dense deployments, and end-to-end problem identification using TEMS Discovery drive testing and advanced trace analysis tools. The team utilized multiple tools including TEMS Discovery, ITK, Business Object, ERTT, VoLTE E2E Analyzer & NEO, Wireshark, and ECO for comprehensive analysis.​

Results: The team won the DFW (Dallas-Fort Worth) KPI improvement challenge, demonstrating superior performance in metrics such as VoLTE call setup success rate, call drop rate, voice packet loss, MOS (Mean Opinion Score), and handover success rate through systematic optimization. Specific improvements included addressing MT RRC re-establishment feature that does not work if cell relation is missing resulting in VoLTE as well as LTE drop, with workarounds implemented showing significant improvement in retainability. DL ICIC feature showed good improvement in VoLTE retainability, and QCI 1 specific “b2Threshold2RscpUtraOffset” helped improve VoLTE retainability as well as SRVCC performance, with offset value of 10dB showing good results. The problem could be severe when layering strategy does not allow handover from one carrier to other (IFHO), with workarounds implemented to reduce impact. This real-world optimization experience validates how comprehensive VoLTE training covering LTE RAN planning, IMS call flows, SIP/Diameter troubleshooting, codec optimization, QoS tuning, SRVCC procedures, and systematic KPI analysis directly enables engineers to deliver measurable service quality improvements at scale across diverse operator environments.​

Be inspired by how SK Telecom, Telefónica Deutschland, and Ericsson deliver millions of high-quality VoLTE calls every day. Join the Rcademy Voice Over LTE (VoLTE), SIP and the IMS course to gain the IMS architecture and signaling skills that power reliable voice networks.