The present document specifies service requirements for Combining CS and IMS services using a CS speech or CS multimedia call in association with an IMS session. The IMS session may consist of one or more IMS services. Requirements for the following capabilities are included: - Radio capability exchange. - Terminal capability exchange. - E.164 number exchange. - Adding IMS session to an ongoing CS call. - Adding a CS call to an ongoing IMS session. - Supplementary services as they relate to CSICS. It is intended that the capabilities defined herein for CSICS shall support interoperability between different operator networks@ and roaming.