No voice sound from the server and from the client

i cant hear the caller and the caller cant hear me, but when i connected the server straight to the adsl modem(before, the server was connect to wr841N router), all was ok, but when i connect the server to the wr841N router, the logs show that everything is ok but there is no voice.
i did DMZ to the server ip, but nothing helped…
also stoped the iptables of linux
my outgoing setting of the trunk is:

disallow=all
allow=ulaw
canreinvite=no
context=from-trunk
dtmfmode=rfc2833
host=provider sip trunk
incominglimit=1
nat=yes
port=provider sip port
qualify=yes
type=friend

please help me…