Hello,
your messages about CISCO ATA reminds me that I have also one (a preproduction unit originally made by Komodo), so I upgraded firmware to 2.15 and tried it behind a symmetric NAT. It works, I had to enable portforwarding for signalling and media and setup properly outer address of the NAT box in the phone.
Once you finish your NAThelper module, I would like to give it a try and see if it works without modifications of the NAT box. So I can help you to test the code if you want.
regards, Jan.