How does fax detection on an incoming route work?

I set up my only incoming route to detect faxes and set the proper destination for detected fax versus non-fax.

When I send a fax from a physical machine coming in through a DID via IAX2, the WARP never seems to detect the fax. I have tried letting it ring-ring-ring, and answering and listening to the sending machine beep for a while.