VoLTE (Voice over LTE) is the 3GPP-standardised method for delivering voice calls over a 4G LTE network using the IP Multimedia Subsystem (IMS). Defined in GSMA IR.92, it carries voice as RTP packets over a dedicated QCI 1 bearer, delivering HD audio via the AMR-WB codec and eliminating the need to fall back to 2G or 3G for voice.
LTE is a data network. It does not carry voice in the same way that 2G or 3G networks did. Before VoLTE was deployed, a 4G phone making a voice call used a mechanism called Circuit Switch Fallback (CSFB): the handset dropped back to the 2G or 3G network, completed the call on a circuit-switched bearer, then returned to 4G for data. This added 6 to 8 seconds to call setup and dropped the data connection for the duration of the call.
VoLTE solves this by carrying voice as IP packets over the LTE network itself, using the IMS (IP Multimedia Subsystem) as the signalling and media control layer. The call never leaves 4G.
The IMS is the architectural framework that manages SIP-based multimedia sessions including VoLTE voice, video calls, and Wi-Fi calling (VoWiFi). It is standardised in 3GPP TS 23.228. The IMS handles call registration, routing, supplementary services (forwarding, voicemail), and interworking with legacy PSTN networks.
VoLTE calls use the AMR-WB (Adaptive Multi-Rate Wideband) codec, commercially marketed as HD Voice. AMR-WB captures audio from 50 Hz to 7,000 Hz at 12.65 kbps. The legacy AMR-NB codec used on 2G and 3G voice captured only 300 Hz to 3,400 Hz. The result is noticeably clearer speech with more natural low-frequency presence and reduced artefacts from background noise.
| Codec | Network | Frequency range | Bit rate | Quality |
|---|---|---|---|---|
| AMR-NB | 2G / 3G | 300 Hz – 3,400 Hz | 4.75-12.2 kbps | Narrowband |
| AMR-WB (HD Voice) | 4G VoLTE | 50 Hz – 7,000 Hz | 6.6-23.85 kbps | Wideband |
| EVS Wideband | VoLTE / VoNR | 50 Hz – 8,000 Hz | 7.2-128 kbps | Super-wideband |
| EVS Fullband | VoNR | 20 Hz – 20,000 Hz | 9.6-128 kbps | Fullband |
Because VoLTE carries voice as packets on the LTE bearer rather than switching to a separate circuit, the LTE data connection remains active during a call. A device on a VoLTE call can continue sending and receiving data at full LTE speed. This matters particularly for:
| Mechanism | Network used | Call setup | Data during call | Voice quality |
|---|---|---|---|---|
| CSFB | Falls to 2G/3G | 6-8 seconds | Drops to 2G/3G | Narrowband |
| VoLTE | 4G LTE (IMS) | Under 2 seconds | Full LTE speed | HD (AMR-WB) |
| OTT (WhatsApp, etc) | Data (any) | Variable | Full speed | Variable |
| VoNR | 5G SA (IMS) | Under 1 second | Full 5G speed | HD/Super-WB |
The 3GPP (3rd Generation Partnership Project) defined the IMS framework that underpins VoLTE in 3GPP TS 23.228 and the related specifications. The GSMA (GSM Association) produced IR.92 – the IMS Profile for Voice and SMS – which defines the specific subset of 3GPP specifications that operators must implement to ensure interoperability between VoLTE handsets and networks. Without IR.92, different operators’ VoLTE implementations would not interwork reliably.