Question: What is the specific format of a SIP URI for an Unknown User Identity in telecom networks, as per 3GPP specifications?
Option 1: sip:anonymous@private.domain
Option 2: sip:unregistered@public.network
Option 3: sip:unknown@unknown.invalid
Option 4: sip:guest@temporary.account
Explanation: The correct format for an Unknown User Identity's SIP URI, as adhering to standards, requires both the user part and domain part to convey the absence of identifiable information. Thus, the user part is set to 'unknown' and the domain part to 'unknown.invalid', ensuring consistency across networks when encountering unidentified users, crucial for interoperability in IP Multimedia Core Network subsystems.