How to Debug NAT Issues, Call Cutoffs and Dropping and Constant Phone Ringing!?

General Help
1

Hello!

I’m having a problem with a particular client and I’m at my wits end. I believe it’s a NAT issue, but I’m not sure how to fix it exactly.

Here are the issues they are facing:

  1. Calls cut off after 15 minutes (NAT Issue)
  2. Call transfers don’t work (NAT Issue?)
  3. Phones in a queue and ring group continue ringing even after call from the PSTN hangs up (Definitely NAT)
  4. BLF doesn’t work on their phones (NAT?)

Their offices uses Grandstream 2160’s, 3240’s and one 3275.

All these phones are remote and my server is remote as well. Here’s what I’ve done on my side so far:

  1. My Asterisk SIP Settings have NAT=yes, the external IP and local networks fields are filled in correctly with my IP address and gateway and subnet
  2. My IP is a static IP from my colocation so much IP Configuration=Static IP
  3. I’m using the default fail2ban and IPTables settings from the FreePBX 11 distro
  4. Reinvite Behaviour=no
  5. RTP Port Ranges= 10000(rtpstart) 20000(rtpend) (I don’t use Webmin)
  6. The extensions themselves have nat=yes

At the client site they have a NetGear R8000 router. It has the SIP ALG helpers which I disabled as soon as I got on site, but calls are still dropping and issues are still rampant. Also, I’m getting retransmission errors like the following as well:

[2015-03-18 16:33:23] WARNING[5256]: chan_sip.c:4024 retrans_pkt: Retransmission timeout reached on transmission [email protected] for seqno 106 (Critical Request) -- See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
Packet timed out after 6401ms with no response
    -- Stopped music on hold on SIP/2610-00002fa0

Everything is SCREAMING NAT ISSUES!! What are my next steps and how do I get calls to stop dropping, transfers to working, and the incessant ringing of extensions in ring groups or queues to stop?! Getting the client to purchase any additional hardware is not possible as I already made them purchase a different router (I told them Cisco, but they got this Netgear instead -.-)

Any help would be greatly appreciated!

System Info:
FreePBX Distro FreePBX-5.211.65-14-x86_64-Full-1402716429, so that would be:

Asterisk 11.10.2
FreePBX 2.11.0.38