How to come up with the numbering of internal telephony?

Question

Telephony

Good day comrades! There is a departmental IP PBX (FreePBX). There is a need to call the higher organization through the IAX trunk using short numbers (such as 5500), in turn, they call us and they are waiting for a call from us (numbers like 8800-8899, that is, one hundred numbers). BUT! there are more than 250 subscribers in our organization, how can we get out of the situation? Thank you!