1. Do you have your phone system route different DID numbers to different extension. Then assign a dedicated modem to each extension?

No I have 4 modems in my fax server 1 dedicated outbound, 2 inbound/outbound, 1 dedicated inbound.

The system then forwards the call to one of the 3 inbound modems just like it does with VoiceMail extensions (I have a Comdial FXII system). And it also forwards the DID digits.

2. Do you have your phone system somehow pass through the DID numbers via a DTMF tone or some other method that your fax server can recognize?

Yes via DTMF.

3. Do you used the SMDR data of your phone system to somehow match up to the received fax in order to route it?

NO.

HTH,
-David