From extensions.conf:

[inbound]
exten => 998,1,Goto(fax,1)


[fax]
exten => s,1,answer()
exten => s,n,wait(1)
exten => s,n,Set(LOCALSTATIONID=${IPKALL})
exten => s,n,Set(FAXRECIPIENT=jblack)
exten => s,n,AGI(receive_fax,receive)
exten => h,1,Set(MY_CALLERIDNUM=${CALLERID(num)})
exten => h,2,DeadAGI(receive_fax,deliver)