The IP multimedia subsystem (IMS) became one of the popular technical solutions which operators use to migrate from circuit switching networks to packet switching networks. Initially the IMS platform was created at the request of mobile operators, but nowadays fixed operators are also actively implementing the IMS platform. Furthermore, the regulations of some countries/regions demand operators to continue providing the set of services that were available over the former circuit switching technologies (e.g., signalling system 7, intelligent networks, etc.). There are different implementations of the SIP-IMS protocols. Operators expressed their interest in establishing the universal requirements that end-user equipment and network equipment should meet. The version number, v.1, indicates that this is version one of Recommendation ITU-T Q.3403, and that it relates to Release 10 of the relevant 3GPP/ETSI standard. This Recommendation endorses ETSI TS 124 229 V10.20.0 (2015-07) "IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3". It contains requirements for establishing the basic call using the IMS platform at the user and network sides.