Gosh, yes it's a bit of a mess... The PSAP (E911 end point) likely will receive an MDN/MSISDN (10 digit number you dial for NANP networks) - this is so they ca

Gosh, yes it's a bit of a mess... The PSAP (E911 end point) likely will receive an MDN/MSISDN (10 digit number you dial for NANP networks) - this is so they can call back if the call is dropped. E911 is a special service, so in the case of deactivated/missing SIM cards, the carrier assigns a temporary MSISDN for duration of the call when the UE exits E911 mode - there are actually of regulatory and carrier requirements around E911 mode. E911 Phase 2 required not only the DN, but also if possible the location of the device - whether thru Cell Base Station Triangulation (if possible) or GNSS with a LAT/LONG based on coarse/fine location info. In any case, as @tjohns mentions, IMSI/IMEI are not typically used outside of the servicing network.