3GPP 23.501 v20.2.0 — the document's own text

4.4.3 IMS support

Taught in 21. Emergency calls, messages, voice and location (The 5G system architecture, in depth).

IMS support for 5GC is defined in TS 23.228 [15].

The 5G System architecture supports N5 interface between PCF and P-CSCF and supports Rx interface between PCF and P-CSCF, to enable IMS service. See TS 23.228 [15], TS 23.503 [45] and TS 23.203 [4].

NOTE 1: Rx support between PCF and P-CSCF is for backwards compatibility for early deployments using Diameter between IMS and 5GC functions.

NOTE 2: When service based interfaces are used between the PCF and P-CSCF in the same PLMN, the P-CSCF performs the functions of a trusted AF in the 5GC.