[sip-comm] problem to set up a connection when both enpoints are behind the NAT



I have a problem when both sip-communicators are behind the FW. Both
endpoints are registered to iptel.org. When the callee answers the call,
it stays in 'Alerting' status. The caller's status shows 'connected'. A
network trace show the caller got the '200 OK, with session decription'
from other side and sent request 'ACK
sip:;nat=yes;transport=udp' to iptel.org. The caller
tries to send this ACK multiple times but none get to the callee. And
eventually the callee will timeout and abort the attempt to setup the
connection. I tried this with the callee outside the firewall and the
connection can be set up on both endpoints. So does anybody have any
idea what went wrong? Or where I should look into (config settings...?).