As per my knowledge, When an UE tries to attach to network with attach type "Emergency", it may not have IMSI. In that case, UE shall include IMEI in attach request. As soon as MME receives this emergency attach request message without IMSI, it would not inquiry to HSS for Authentication-Information-procedure or Update-Location procedure since for both the messages, IMSI is necessary. MME checks with EIR for the validity of IMEI. Once IMEI is authenticated or based on the operator policy response (in case of IMEI authentication failure) , MME proceeds further.
