[Asterisk-Users] nat problem

[Webn1]

hello,

i have trouble with nat + sip outgoing call.when make an outgoing call to a
sip gateway, i have no sound.

i have 2 sip gateway, one is asterisk.

asterisk is on public ip and private ip
other sip gateway is on public ip
phone are cisco and grandstream on private ip on the same subnet as
asterisk.
phone are connected by sip to asterisk (i have try with or without nat=yes)

incoming call from sip gateway to asterisk : no problem
outgoing call from asterisk to sip gateway : no sound

no sip error
if i put a phone on public ip, incoming and outgoing call work fine

i haven't make any port translation since phone and asterisk are on the same
private subnet.
phone register on the private ip of asterisk

i have zaphfc connected to isdn and incoming and outgoing call work fine
(with phone on public or private ip)

any idea what's wrong or try to fix this problem ?

thanks for help