Question: In an IMS (IP Multimedia Subsystem) network supporting multimedia telephony, how are voice/video calls originating from the PSTN (Public Switched Telephone Network)/CS (Circuit Switched) domain identified for multimedia telephony service?
Option 1: By analyzing the call's SIP (Session Initiation Protocol) headers for specific codec information.
Option 2: Through a dedicated SS7 (Signaling System No. 7) signaling link reserved for multimedia telephony.
Option 3: By marking them with a communication service identifier associated with the multimedia telephony communication service.
Option 4: Based on the caller's subscription to a specific multimedia telephony package, checked during call setup.
Explanation: This is correct because, according to IMS principles for identifying services, associating a unique communication service identifier with multimedia telephony enables straightforward identification and handling of such calls, even when they originate from outside the IP domain (e.g., PSTN/CS). This ensures seamless integration and proper routing or treatment of these calls within the IMS network.