IMS

IMS is the SIP-based service layer that delivers voice, video and messaging over packet networks, and its biggest virtue is that it's access-agnostic. The same core — P/I/S-CSCF, HSS/UDM, application servers — serves a call whether the user is on LTE (VoLTE), 5G NR (VoNR), or Wi-Fi calling. Only the access and the bearer underneath change; the call logic doesn't. That's why an operator could add VoNR without rebuilding voice from scratch — they reused the IMS they already ran for VoLTE.

It predates 5G by a long way; the architecture came out of the 3GPP work in the 2000s and was the foundation of VoLTE. The pieces fit together as a chain: SIP for signalling, RTP for media, the CSCFs for routing and registration, the TAS/MMTel for actual features, and Diameter or HTTP-based interfaces to the subscriber database. For someone new to it, the mental model is "a standardised IP telephony network bolted onto whatever mobile or fixed access the operator has" — that reuse is the entire reason it exists. SIP in VoLTE and VoNR follows registration and call setup message by message, and the IMS SIP 401 loop is a real capture of a registration that never completes — free with an account.