Initial UE Message on the S1 interface has a RRC Establishment IE, which will indicate whether the call is an emergency call or other priority. Consequently the MME can take appropriate action e.g. decide whether to accept USIM-free calls/IMEI identity allocate ARP to the E-RAB(s), and anything it wishes to do.
Some countries allows call to be made without a USIM present, in which case the NAS identity presented by the UE will be the IMEI.