The "combination of CS and IMS services" (CSI) is essentially a combination of existing CS and IMS services, i.e. mechanisms and procedures for the IMS part of the CSI session apply according to TS 23.228. The UE presents the CS call and IMS session within one context to the user. To facilitate this, the following capabilities shall be provided:

1. Exchange of information related to the current radio environment;
2. Exchange of terminal capability information;
3. Addition of an IMS session to an ongoing CS call;
4. Addition of a CS call to an ongoing IMS session.
5. Network support for establishing multimedia sessions between a UE that uses IMS origination and a UE that uses CSI termination.

CSI services can be provided:

1. Between two CSI capable UEs that both use CSI origination and CSI termination;
2. Between a UE that uses IMS origination and a UE that uses CSI termination.

The service requirements of combining IMS and CS services are described in TS 22.279.