IVR and incoming fax calls

Using Asterisk 1.4.19:

I want to be able to detect incoming fax calls in my IVR menu and route them to my fax machine (connected to HandyTone 503). I tried inserting NVFaxDetect() right after Answer() in the IVR menu but then all I get is busy signal and the IVR menu will not play. How can I set things up so that Asterisk will automatically route incoming fax call during IVR menu?