in iax.conf
[iaxfwd]
type=user
context=fromiaxfwd
;auth=rsa
;inkeys=freeworlddialup
deny=0.0.0.0/0.0.0.0
permit=65.39.205.0/255.255.255.0

in extensions.conf
[fwd-trunks]
exten => _393.,1,SetCallerId,${FWDCIDNAME}
exten => _393.,2,Dial(IAX2/${FWDNUMBER}:${FWDPASSWORD}@iax2.fwdnet.net/${EXTEN:\3},60,r)
exten => _393.,3,Congestion

[fromiaxfwd]
exten => 431505,1,Goto(local-access,200,1)
exten => 431505,2,Hangup

-- 
the error
Jun 14 14:44:40 WARNING[1142106560]: chan_iax2.c:5107 socket_read: Call rejected by 65.39.205.121: No such context/extension
 when calling out