Question: In a SIP INVITE request for an unauthenticated emergency session, how is the IMEI based identity formatted in the P-Preferred-Identity header field by a User Equipment (UE)?
Option 1: imei:<encoded_imei_value> without a domain part
Option 2: sip:user@domain where "user" contains the encoded IMEI and "domain" is the home network domain name
Option 3: tel:+<encoded_imei_value> with an optional parameter for the home network
Option 4: urn:imei:<encoded_imei_value> without referencing a specific domain
Explanation: According to telecom standards, the IMEI must be formatted in a way that's both identifiable and traceable to a network. The sip:user@domain format, where the "user" part encodes the IMEI and the "domain" part specifies the home network domain name, meets this requirement, aligning with SIP URI standards for identity verification and routing within telecom networks.