Block Faxes on DID's not setup for use


I have a weird issue that I need some help on. My setup is mostly a fax to email service for our members with 12 phones. We have 1,000 DID’s for the entire service on a PRI. What I have found is that people are faxing the DID numbers that are not setup to receive faxes. I am using NVFax for detection. If I call the number, I get a message that the number is not in service. If I fax something to that same number the fax will start to be received and will often crash Asterisk.

Any idea on how to configure FreePBX so faxes are not received on numbers that are not in service? One solution I have come up with is to create an entry for all the numbers not is use and set it not to detect the fax, but that solution seems odd. Any help or advice is appreciated.