Thank you All.
This is what I have done so far the but the issue still exist.
I configure the pbx to initiate PPPoE and connected directly to the modem on the WAN, I got the public IP address on it directly. After that I still continue to face the same problem incoming call drop exact after 32 secs.
interestingly if I configure by inbound rule to send the incoming call to a register extension / phone the call will continue to work for more 32 seconds with no problem.
I am now more convince the issue is with the new update. Because this issue was not there when I was using the old version.
I have not idea what to do now.
Can someone please advise. I just regret upgrading and now have no idea how to get this issue resolve.