VoLTE

VoLTE (Voice over LTE) is the standard way to make phone calls over a 4G LTE network. Instead of switching to a 2G or 3G circuit-switched network for each call, the device carries voice as IP packets over LTE, with call control handled by the operator’s IMS (IP Multimedia Subsystem). The GSMA’s IR.92 profile defines the minimum feature set for interoperable VoLTE devices and networks.

How a VoLTE call works

1. IMS connection: after attaching to LTE, the device opens a second data connection on the IMS APN, usually ims, carried on a bearer reserved for signaling (QCI 5).
2. IMS registration: the device registers with IMS using SIP, authenticated with credentials from the SIM or eSIM profile.
3. Call setup: a SIP INVITE travels through the IMS core to the called party, and the network sets up a dedicated guaranteed-bit-rate bearer for the voice media (QCI 1).
4. Media: speech flows as RTP packets, typically encoded with AMR-WB (“HD Voice”) or EVS.

Why VoLTE matters

• 2G/3G shutdowns: as networks retire 3G and 2G, devices without working VoLTE lose voice entirely — including emergency calls.
• Quality: wideband codecs and faster call setup improve on circuit-switched voice.
• IoT: LTE-M supports VoLTE, which matters for alarm panels, elevator phones and emergency buttons.
• 5G: in 5G NSA voice stays on VoLTE; in 5G SA calls use VoNR or fall back to VoLTE.

VoLTE and roaming

VoLTE roaming requires the home and visited networks to agree on how IMS traffic is handled — commonly home routing, where IMS traffic returns to the home network. GSMA IR.23 requires partners to verify home routing before running the IR.25 VoLTE roaming tests (see IREG). Where VoLTE roaming is not in place, roamers fall back to 3G or 2G voice — or have no voice where those networks are gone.

Why it matters for testing

A device can attach to LTE and pass data tests yet fail voice because IMS registration fails, the IMS APN is missing, or the roaming partner does not support VoLTE. Voice tests should report IMS registration state, bearer type, call setup time and outcome. SimCheck.ai’s Voice Call Check places a real call and reports IMS state, bearer (VoLTE, VoNR or circuit-switched), setup time and outcome — see VoLTE and voice testing.