Question: What is the specific format of the SIP URI for an Anonymous User Identity in telecom networks, as per 3GPP specifications?
Option 1: sip:private@hidden.domain
Option 2: sip:anonymous@operator.network
Option 3: sip:user@anonymous.com
Option 4: sip:anonymous@anonymous.invalid
Explanation: The correct format for an Anonymous User Identity's SIP URI, as specified by 3GPP, requires both the user part to be "anonymous" and the domain part to be "anonymous.invalid". This specific formatting ensures anonymity while adhering to SIP URI standards (as per IETF RFC 3261), making **Option 4** the correct choice. This format is crucial for maintaining user privacy in interworking scenarios between IP Multimedia (IM) Core Network subsystems and Circuit Switched networks.