Question: What is the format of a SIP URI for an Unavailable User Identity in a telecom network?
Option 1: sip:user@available.valid
Option 2: sip:unavailable@ims.mncXXX.mccYYY.3gppnetwork.org
Option 3: sip:unavailable@unknown.invalid
Option 4: sip:user@domain.tld
Explanation: The correct format of a SIP URI for an Unavailable User Identity is indeed sip:unavailable@unknown.invalid, as it follows the specified structure with the user part being unavailable and the domain part being unknown.invalid. This ensures that the Unavailable User Identity can be properly identified and handled in telecom networks.