Fax auto detection

On our server in Memphis we use Fax detection on the inbound routes. The problem we are having is not system wide and doesn’t always happen.
When you call the DID sometime the call is passed to the phone and sometimes you will hear the fax tones.
We are running:
FreepBX verison 1.811.210.57-1

Any ideas on what would be causing the issue?

We use SIP.

Fax detection has not worked reliably since Asterisk 1.4, I always use a dedicated line for fax routing.

Also are you using pots lines or SIP trunks for your faxing?