[asterisk-users] asterisk and sip nat problems

[mike]

Hi all !

i'm having a strange issue with an asterisk box behind a firewall
i'm trying to answer a sip call made to an asterisk box with a public ip
from another asterisk box behind a firewall

on the natted box i've put

externip=195.110.XXX.XXX
localnet=10.1.1.0/255.255.255.0

and on the phone context i've added
nat=yes


the call starts from the natted box, 
the other phone rings
on phone pickup, from the public asterisk the message is the following:
Attempting native bridge of SIP/83.211.XX.XXX-0819b588 and SIP/1-f3d0
after 3 seconds, the natted box prints the following:
No one is available to answer at this time

what do you think it's missing ?

thank you very much
.mike