VoLTE Glossary

Technical reference definitions for VoLTE, IMS, and related cellular voice technology. All terms are based on 3GPP and GSMA standards.

AMR-NB

Adaptive Multi-Rate Narrowband. The voice codec used on 2G and 3G networks. Frequency range 300 Hz to 3,400 Hz. Bit rate 4.75 to 12.2 kbps. Replaced by AMR-WB for VoLTE.

AMR-WB

Adaptive Multi-Rate Wideband. The HD Voice codec used for VoLTE calls. Frequency range 50 Hz to 7,000 Hz. Bit rate 6.6 to 23.85 kbps at the highest mode. The default VoLTE codec defined in GSMA IR.92.

ARC

Alarm Receiving Centre. A monitoring facility that receives alarm signals from intruder, fire, and personal alarm systems. ARC signalling typically uses IP-based protocols or PSTN/cellular voice paths.

ATA

Analogue Telephone Adapter. A device that converts analogue voice signals (from an RJ11 interface) to IP or cellular for transmission. Used as a PSTN replacement endpoint for legacy telephony equipment.

CSFB

Circuit Switch Fallback. The mechanism by which a 4G LTE device drops to a 2G or 3G network to complete a voice call, because the 4G network does not have VoLTE deployed. No longer functional in the UK – all 3G networks closed in 2024.

EPC

Evolved Packet Core. The 4G LTE core network. Comprises the MME (Mobility Management Entity), SGW (Serving Gateway), PGW (Packet Data Network Gateway), HSS, and PCRF. The 5G SA equivalent is the 5GC (5G Core).

eSRVCC

Enhanced Single Radio Voice Call Continuity. An improved version of SRVCC that reduces the interruption during handover from VoLTE to a circuit-switched network to under 300ms. Defined in 3GPP Release 10.

EVS

Enhanced Voice Services. An advanced voice codec defined in 3GPP TS 26.441. Supports narrowband, wideband, super-wideband (up to 14.4 kHz), and fullband (up to 20 kHz) audio. Used in VoNR and some VoLTE implementations. Superior packet loss resilience compared to AMR-WB.

GBR bearer

Guaranteed Bit Rate bearer. An LTE EPS bearer that reserves a specific bandwidth for the connection. VoLTE voice calls use a GBR bearer with QCI 1 to ensure the voice packets have guaranteed bandwidth and priority.

HD Voice

The commercial marketing name for calls using the AMR-WB codec. Used by all four UK operators to describe VoLTE call quality. Not a separate standard – it is AMR-WB.

HSS

Home Subscriber Server. The central subscriber database in the LTE/IMS network. Stores subscriber profiles, authentication vectors, IMS identities (IMPI/IMPU), and service entitlements.

I-CSCF

Interrogating Call Session Control Function. An IMS element that queries the HSS to determine which S-CSCF should serve an incoming registration or session request from a subscriber.

IMPI

IMS Private User Identity. A permanent identifier assigned to a subscriber within the IMS, formatted as an NAI (Network Access Identifier). Used for authentication – not shared with other parties. Stored on the ISIM.

IMPU

IMS Public User Identity. The addressable identifier used to route calls and messages to a subscriber, formatted as a SIP URI (sip:+447700900123@operator.com) or tel URI. One subscriber may have multiple IMPUs. Stored on the ISIM.

IMS

IP Multimedia Subsystem. The architectural framework that manages SIP-based multimedia sessions including VoLTE voice, video calls, Wi-Fi calling, and messaging. Defined in 3GPP TS 23.228.

ISIM

IMS Subscriber Identity Module. A logical application on the UICC that stores IMS credentials: IMPI, IMPU, and IMS service profile entitlements. A SIM must carry an ISIM to support VoLTE. Standard IoT data SIMs typically do not include an ISIM.

MGCF

Media Gateway Control Function. An IMS element that interworks VoLTE calls with the PSTN or legacy circuit-switched networks. Performs SIP-to-ISUP protocol translation to route calls to non-IMS destinations.

P-CSCF

Proxy Call Session Control Function. The first IMS contact point for a device registering for VoLTE services. Handles SIP registration forwarding, QoS authorisation for voice bearers, and compression of SIP messages.

PCRF

Policy and Charging Rules Function. The LTE element that authorises QoS policies for EPS bearers. For VoLTE, the PCRF authorises the GBR bearer with QCI 1 for each call, ensuring the voice stream has guaranteed bandwidth and priority.

PSTN

Public Switched Telephone Network. The UK’s copper-wire analogue telephone infrastructure. Being switched off by January 2027 as part of BT Openreach’s Digital Voice programme.

QCI

QoS Class Identifier. A value that defines the quality of service parameters for an LTE EPS bearer. VoLTE uses QCI 1 (GBR, priority 2, packet delay budget 100ms, packet error rate 10^-2). QCI 5 is used for IMS signalling.

RTP

Real-time Transport Protocol. The protocol used to carry voice and video media packets in a VoLTE call. RTP packets travel over UDP over the dedicated QCI 1 bearer between the two devices.

S-CSCF

Serving Call Session Control Function. The central IMS registration and session routing function. Manages subscriber registration state, applies service profiles, and routes SIP sessions to the TAS for supplementary services.

SIP

Session Initiation Protocol. The signalling protocol used to establish, modify, and terminate VoLTE sessions. SIP INVITE initiates a call; SIP BYE ends it. The SIP dialogue runs between the UE and the IMS via the P-CSCF.

SRVCC

Single Radio Voice Call Continuity. The mechanism that hands an active VoLTE call to a 2G or 3G circuit-switched network if LTE coverage is lost. With 3G networks closed in the UK, SRVCC to 2G remains theoretically available on networks that retain 2G.

TAS

Telephony Application Server. An IMS application server that implements supplementary services for VoLTE: call forwarding, call waiting, call hold, conference calling, and voicemail access.

UICC

Universal Integrated Circuit Card. The physical smart card that contains SIM applications. A VoLTE-capable UICC contains both a USIM (for LTE authentication) and an ISIM (for IMS authentication).

USIM

Universal Subscriber Identity Module. The application on a UICC that handles LTE and UMTS authentication. Contains the IMSI and authentication keys. Present on all modern SIM cards.

VoLTE

Voice over LTE. The 3GPP-standardised mechanism for carrying voice calls over a 4G LTE network using the IMS. Defined in GSMA IR.92. Uses SIP for signalling, RTP for media, AMR-WB for audio encoding, and QCI 1 for the dedicated voice bearer.

VoNR

Voice over New Radio. The 5G equivalent of VoLTE. Carries voice over the 5G NR air interface using the 5G Standalone core (5GC) and IMS. Supports the EVS codec. Requires 5G SA deployment. See vonr.co.uk for full detail.

VoWiFi

Voice over WiFi. IMS-based voice calls delivered over a WiFi connection via an IPsec tunnel to the operator’s network. Uses the same IMS and ISIM credentials as VoLTE. Useful in areas with poor cellular coverage but available WiFi.