This Recommendation specifies the requirements for providing the interworking between the IP multimedia (IM) core network (CN) subsystem and circuit switched (CS) networks. The Recommendation endorses ETSI TS 129 163 V10.19.0 (2016-01) Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks (3GPP TS 29.163 version 10.19.0 Release 10).