Question: In a Limited Service State, when an IMSI is not available or authenticated during an emergency registration, what format does the Emergency NAI take if only the UE's IMEI (219551288888888) is available?
Option 1: imei-219551288888888@emergency.services
Option 2: imei:219551288888888@sos.invalid
Option 3: imei219551288888888@sos.invalid
Option 4: IMEI_219551288888888@reserved.emergency
Explanation: According to the specified format for an Emergency NAI in a Limited Service State when only the IMEI is available, the correct structure is imei<IMEI>@sos.invalid. This is because .invalid is a reserved top-level domain used here since the NAI does not require resolution for routing. The provided IMEI (219551288888888) directly follows imei, matching the format specified in the guidelines.