Question: In an IMS (IP Multimedia Subsystem) scenario, when an I-CSCF (Interrogating-Call Session Control Function) receives an INVITE request with a user identity in E.164 number format within a SIP URI (Session Initiation Protocol Uniform Resource Identifier), what action must the I-CSCF take before sending a DX_SLF_QUERY to the SLF (Subscription Locator Function)?
Option 1: Forward the INVITE request directly to the HSS (Home Subscriber Server) for authentication.
Option 2: Translate the E.164 number into an IMSI (International Mobile Subscriber Identity) format.
Option 3: Convert the SIP URI with the E.164 number from "user=phone" parameter format to Tel: URI format as per IETF RFC 3966.
Option 4: Reject the INVITE request due to unsupported user identity format.
Explanation: When the I-CSCF receives an INVITE with a user identity in E.164 format within a SIP URI, it must first translate this into Tel: URI format according to IETF RFC 3966 before querying the SLF. This ensures compatibility and proper handling of the query by the SLF, highlighting the importance of format standardization in intersystem communications within telecom networks.