J'ai trouvé ça pour configurer OVH dans Asterisk :

# cat /etc/asterisk/sip.conf

[general]
context=forfait-ovh
bindport=5060
bindaddr=0.0.0.0
srvlookup=yes
register => 0033XXXXXXXXX:password@sip.ovh.net

[forfait-ovh]
type=peer
host=sip.ovh.net
context=ovh-sip
language=fr
insecure=very
username=0033XXXXXXXXX
secret=password
nat=yes
canreinvite=no
dtmfmode=auto
video=no
restrictcid=no
amaflags=default

http://guide.ovh.com/AsteriskEtForfaitOVH