A method for interworking a session by a Mobile Switching Center, MSC, server enhanced for IP Multimedia Subsystem, IMS, Centralized Services, ICS, the method comprising: receiving (11, 602), at the MSC server, a Session Initiation Protocol, SIP, invite message originated by a first user equipment, UE, for establishing a communication session with a second UE involved in a call with a third UE, the call being served in a circuit switched, CS, domain by the MSC server, wherein the SIP invite message is received from an IMS network comprising a call waiting application server, wherein the SIP invite message comprises a MIME body, the MIME body including a communication-waiting indication element inserted by the call waiting application server for indicating that a communication waiting service provided by the call waiting application server is available to interwork with a call waiting service provided by the MSC server, and the MSC server is configured to recognize the communication-waiting indication element within the MIME body of the SIP invite message; in response to receiving, at the MSC server, the SIP invite message and recognizing the communication-waiting indication element, the MSC server: sending (12, 606) to the second UE a CS setup message comprising an information element indicating the available communication waiting service, and storing an indication that the session includes the call waiting application server providing the communication waiting service; receiving a CS message from the second UE, the CS message including one of: a call waiting information; and an indication that the second UE does not support the communication waiting service; if the CS message comprises the call waiting information, sending (16, 612, 618) a SIP response message to the SIP invite message to the IMS network, the SIP response message including message information mapped from the call waiting information; and if the CS message comprises the indication the second UE does not support the communication waiting service, transmitting an unsupported media type response for the first UE to the IMS network. receiving (11, 602), at the MSC server, a Session Initiation Protocol, SIP, invite message originated by a first user equipment, UE, for establishing a communication session with a second UE involved in a call with a third UE, the call being served in a circuit switched, CS, domain by the MSC server, wherein the SIP invite message is received from an IMS network comprising a call waiting application server, wherein the SIP invite message comprises a MIME body, the MIME body including a communication-waiting indication element inserted by the call waiting application server for indicating that a communication waiting service provided by the call waiting application server is available to interwork with a call waiting service provided by the MSC server, and the MSC server is configured to recognize the communication-waiting indication element within the MIME body of the SIP invite message; in response to receiving, at the MSC server, the SIP invite message and recognizing the communication-waiting indication element, the MSC server: sending (12, 606) to the second UE a CS setup message comprising an information element indicating the available communication waiting service, and storing an indication that the session includes the call waiting application server providing the communication waiting service; receiving a CS message from the second UE, the CS message including one of: a call waiting information; and an indication that the second UE does not support the communication waiting service; if the CS message comprises the call waiting information, sending (16, 612, 618) a SIP response message to the SIP invite message to the IMS network, the SIP response message including message information mapped from the call waiting information; and if the CS message comprises the indication the second UE does not support the communication waiting service, transmitting an unsupported media type response for the first UE to the IMS network. sending (12, 606) to the second UE a CS setup message comprising an information element indicating the available communication waiting service, and storing an indication that the session includes the call waiting application server providing the communication waiting service; receiving a CS message from the second UE, the CS message including one of: a call waiting information; and an indication that the second UE does not support the communication waiting service; if the CS message comprises the call waiting information, sending (16, 612, 618) a SIP response message to the SIP invite message to the IMS network, the SIP response message including message information mapped from the call waiting information; and if the CS message comprises the indication the second UE does not support the communication waiting service, transmitting an unsupported media type response for the first UE to the IMS network.