Question: What format should an instance-id take in SIP registration when an International Mobile Equipment Identity (IMEI) is available, according to 3GPP specifications?
Option 1: urn:ietf:uuid:<uuid-value>
Option 2: urn:gsma:imei:<imeival> where <imeival> contains the IMEI encoded as per RFC 7254
Option 3: sip:<user>@<domain>;instance=<random-string>
Option 4: tel:+<msisdn>;instance-id=<imei-value>
Explanation: When an IMEI is available for a SIP User Agent (UA) performing a registration, the instance-id must uniquely identify the UA using the IMEI in a standardized format. This format, as specified by telecom standards for interoperability and uniqueness, is urn:gsma:imei:<imeival>, where <imeival> is the IMEI encoded according to RFC 7254. This ensures consistency across different devices and networks, facilitating efficient management and identification of user equipment in telecommunication systems.