Please help! I think it is a firewall problem

I just setup an Asterisk server and other places connect to it. For all other places, everything normal, can register, making calls, hearing 2 ways voice… But only in Svenborg office, I can register softphone, if I making call immediately, call can be establish and I can hear 2 ways voice… It is fine as long as I stay in call. But right after hangup the phone, the phone becomes unreachable… I debug and see that the OPTION packet does not get the response… On the next test, just log in the softphone, after a quite fixed time, that phone becomes unreachable also…
I also tried to turn off qualify and make a un-check call to that phone (in svenborg), it does not work neither…
Can someone help me? Honestly, i have never met this issue before… Do you think that because of NAT? I did set on Asterisk sip config: nat, externip and localnet info… Is it the problem from the firewall of svenborg office? if it is… why it is occured, i almost turn off the firewall. I am using 3-COM SuperStack
Thanks in advance
Duc To

If it were the firewall, you wouldn’t be able to make the first call. check your register and reinvite settings.

Hi! Thanks for your reply
If it is because of my sip or sip exten settings, why I can log to it from another network, and let it works for weeks without problems? Both is under NAT

Duc To

Firewalls are “Wonderful” Things. I’m guessing that your Asterisk is set up for the “normal” 10000-20000 range of open UDP ports. What might be happening is that the “working” call hits this range, then some randomness generates a “call” outside of this range which “stops the world”. netstat -ano -p udp might shed some more light on this.