IMS

IMS (IP Multimedia Subsystem) is the part of a mobile operator’s network that controls voice, video and messaging services over IP. Introduced by 3GPP in Release 5 and based on the Session Initiation Protocol (SIP), it is the call-control layer behind VoLTE, VoNR, Wi-Fi Calling and RCS messaging.

How IMS works

IMS separates service control from the radio and packet core. A device first gets an IP connection — usually on a dedicated IMS APN — and then talks SIP to a chain of functions:

Function	Role
P-CSCF (Proxy)	The device’s first contact point; protects signaling and triggers the policy control that sets up voice bearers
I-CSCF (Interrogating)	Finds the right serving function for the subscriber
S-CSCF (Serving)	Registers the user, applies service logic and routes sessions
HSS / UDM	Stores subscriber identities and credentials
Application servers	Supplementary services such as call forwarding, voicemail and conferencing

Authentication typically uses IMS AKA, with credentials from the ISIM or USIM application in the SIM or eSIM profile. The subscriber is addressed by SIP URIs and by a tel URI built from the MSISDN.

IMS registration

Before any call, the device must complete IMS registration: discover the P-CSCF, send a SIP REGISTER, authenticate, and set up security associations. A device that is attached but not IMS-registered can still use data, but it cannot make VoLTE calls; depending on the network and device, it either falls back to circuit-switched voice or has no voice service.

Where IMS is used

• Voice over LTE, over 5G NR in 5G SA, and over Wi-Fi.
• SMS over IP and RCS messaging.
• Emergency calling over IP.
• Roaming voice, where IMS traffic is commonly routed back to the home network.

Why it matters for testing

IMS problems are a common source of “no voice” complaints that data tests never catch: P-CSCF discovery failures, missing IMS APN permissions, ISIM gaps in an eSIM profile, or roaming partners that do not support the required IMS routing. Checking IMS registration state is the first step in any voice test. SimCheck.ai’s voice tests report IMS state alongside call setup time and outcome — see VoLTE and voice testing.