iax.conf

[general]
register => pauoliva:xxx@iaxtel.com
register => 646499:xxxx@iax2.fwdnet.net
	
; Trust Caller*ID Coming from iaxtel.com
[iaxtel]
type=user
context=default
auth=rsa
inkeys=iaxtel
	
; Trust Caller*ID Coming from iax.fwdnet.net
[iaxfwd]
type=user
context=default
auth=rsa
inkeys=freeworlddialup
	
[fwd-gw] ; outbound connections to FWD
type=peer
auth=md5
secret=xxxx
username=646499
qualify=yes
host=iax2.fwdnet.net
disallow=all
allow=ulaw
callerid=\"Pau\"<646499>
	
[iaxtel-gw] ; outbound connections to iaxtel
type=peer
auth=md5
secret=xxx
username=pauoliva
qualify=yes
host=iaxtel.com
disallow=all
allow=ulaw
callerid=\"Pau\"<17007931122>

extensions.conf

[iaxfwd]
exten => _001.,1,SetCallerId,646499
exten => _001.,2,Dial(IAX2/646499@fwd-gw/${EXTEN:3},60,r)
exten => _001.,3,Congestion
	
[fromiaxfwd]
exten => 646499,1,Dial(646499,20,r)
exten => 646499,2,Voicemail,u200
exten => 646499,102,Voicemail,b200
	
[iaxtel]
exten => _1700NXXXXXX,1,Dial(IAX2/pauoliva@iaxtel-gw/${EXTEN}@iaxtel)
exten => _1888NXXXXXX,1,Dial(IAX2/pauoliva@iaxtel-gw/${EXTEN}@iaxtel)
exten => _1877NXXXXXX,1,Dial(IAX2/pauoliva@iaxtel-gw/${EXTEN}@iaxtel)
exten => _1866NXXXXXX,1,Dial(IAX2/pauoliva@iaxtel-gw/${EXTEN}@iaxtel)
exten => _1800NXXXXXX,1,Dial(IAX2/pauoliva@iaxtel-gw/${EXTEN}@iaxtel)

I afegiu això al context de les extensions que vulgueu que puguin utilitzar el servei:

; Permeto rebre trucades usant FWD
include => fromiaxfwd
; Permeto trucar amb iaxtel
include => iaxtel