SIP.conf
register => mylogin:password@ippi.fr
[ippi_incoming] ; configuration des appels entrants depuis ippi
type=peer
host=ippi.fr
context=from_ippi
nat=yes
canreinvite=no
and the extensions.conf :

[from_ippi] ; test appel entrant ippi
exten => s,1,Dial(MGCP/aaln/1@test)