The following info was taken from this location:

http://www.voip-info.org/tiki-index.php?page=NVFaxDetect



Sample Usage (extensions.conf)
[context-incoming]
; Answer and do some detection work
exten => s,1,Answer
exten => s,2,NVFaxDetect()
exten => s,3,Hangup

; If user presses "1", dial main line
exten => 1,1,Dial(SIP/5500)
exten => 1,2,Hangup

; If this is a fax, dial fax line
exten => fax,1,Dial(SIP/5501)
exten => fax,2,Hangup

; If user is talking, send him to Debra
exten => talk,1,Dial(SIP/5502)
exten => talk,2,Hangup


Another Example (extensions.conf)
; ring extensions with fax detect
[macro-faxexten]
exten => s,1,Dial(${ARG1},17) ; wait 3 rings
exten => s,2,Answer
exten => s,3,Playtones(ring) ; play fake ring so caller doesn't wonder whats going on
exten => s,4,NVFaxDetect(4|dt) ; while playing ring sound, detect faxes for 4 seconds (goes to "fax" extension if detected)
exten => s,5,Goto(s-${DIALSTATUS},1) ; if no fax, branch on dialstatus
exten => s-NOANSWER,1,Voicemail(u${MACRO_EXTEN})
exten => s-NOANSWER,2,Hangup()
exten => s-BUSY,1,Voicemail(b${MACRO_EXTEN})
exten => s-BUSY,2,Hangup()
exten => _s-.,1,Goto(s-NOANSWER,1) ; everything else is treated as no answer

; If this is a fax, get it
exten => fax,1,StopPlaytones ; you must do this or it will play ring sounds over your fax
exten => fax,2,SetVar(FAXFILE=/var/spool/asterisk/fax/${DNID}_${UNIQUEID})
exten => fax,3,Goto(fax,s,1)

[fax]
exten => s,1,rxfax(${FAXFILE}.tif)
exten => h,1,DEADAGI(faxtoemail.agi|${FAXFILE}.tif) ; you need to download the faxtoemail script or something similar here.