If you are fully isolated, and they don’t provide a working DNS environment, there may be some hints in: DNS Availability - Phones
Note that, in such an environment, you will have difficulty updating FreePBX and some commercial parts may refuse to work.
If you have a second interface, with connectivity to the real internet, the following may be helpful, although it seems to suggest that you may have to go direct to Linux: Two NICs -- One is configured with Local Lan and Second will be P2P to my Local Service Provider