Pourquoi t’embêter avec des extensions non numériques
pour commencer l'idéal est
SIP.conf

[freephonie]
type = peer
username = xxxx
context = default
language = fr
host = freephonie.net
fromdomain = freephonie.net
callerid =
secret = yyyy
insecure = very
dtmf = auto
disallaw = all
allow = ulaw
allow = alaw
nat = yes
canreinvite = no

[500]; a la place de chambre
type = friend
username = 500
secret = 1408
host = dynamic
context = maison

[501]; a la place de cuisine
type = friend
username = 501
secret = 1408
host = dynamic
context = maison

[503]; a la place de pchc
type = friend
username = 503
secret = 1408
host = dynamic
context = maison

Extension.conf
[default]
exten => _.,1,GoTo(maison,599,1)
exten => s,1,GoTo(maison,599,1)

[maison]
;un ring group
exten => 599,1,Dial(SIP/500&SIP/501&SIP/502,30)
exten => 599,n,hangup()

exten => _5X.,1,Dial(SIP/${exten}) ;
exten => _X.,1,Dial(SIP/freephonie/${EXTEN})


Tout devrais marcher avec ça :-)