The only way to page out that amount of phones is with a BlazeCast server, otherwise you are limited by the page group size in the Mitel.
Once possibility is you could see if the Cyberdata paging server could register to the Mitel as a SIP device, then dial the group paging code, but you are still limited to the page group limits of the Mitel 3300 (32 or 64 devices depending on software revision iirc), so this might not be an acceptable solution even if it works.