Question: What is the specific format of the SIP URI for an Unavailable User Identity in telecom networks, as per 3GPP specifications?
Option 1: sip:anonymous@private.domain
Option 2: sip:unavailable@unknown.invalid
Option 3: sip:unreachable@public.network
Option 4: sip:offline@temporarily.unavailable
Explanation: The correct format for an Unavailable User Identity's SIP URI, as specified by 3GPP, is with the user part as 'unavailable' and the domain part as 'unknown.invalid'. This specific formatting ensures consistency across telecom networks when indicating a user's unavailability. This aligns with standards for interoperability and clear communication of user status in IP Multimedia subsystems and beyond.