Asterisk not accepting acknowledgement and call drops

We have two asterisk servers running on a x86_64 running Linux. One of them works fine whereas other drops incoming calls with a chan_sip.c:2964 retrans_pkt: Maximum retries exceeded on transmission followed by chan_sip.c:2991 retrans_pkt: Hanging up call E1EC561B@X.X.X.X - no reply to our critical packet (see doc/sip-retransmit.txt).

Now i have collected packet captures of a working and non-working call as well as sip debugs for both. Packet captures tell me that ACK is reaching Asterisk from Service Provider and when i look at SIP Debugs, i see that ACK is not making it to this layer in SIP Stack or no inbound ACK for a transaction in debug trace. So i thought this could be an issue with iptables and i disabled iptables to see what it does but the problem still persists.

To be specific when an incoming call comes it is answered by auto-attendant and when you enter the extension that’s when you hear no audio and call eventally drops. I compared on my configs on both the asterisk servers but to no avail.

I am out of ideas on this one. Any one has any thoughts or might have run into similar issue. Please help me!!

I had this problem from a remote site over a vpn connection. I had to add the ip address range of the remote vpn site to the sip_general_additional.conf (i.e. localnet=

Hope this helps