Cisco Emergency Responder caches a mapping between the caller's true extension and the Emergency Location Identification Number (ELIN) you define for an Emergency Response Location (ERL). If more calls get made than the number of ELINs you define for an ERL, Cisco Emergency Responder must re-use these numbers, and it overwrites the original caller's extension.
You can view the call history to determine the extension of the original caller.
If this does not address the problem, check the configuration of the Public Safety Answering Point (PSAP) callback route point in Cisco CallManager and Cisco Emergency Responder. To resolve this issue, refer to these documents: