Question about "timeout" problem with phone

I am running a@h 2.8 on a data t1 line. My phones are all VOIP at a location other than where the T1 is located. The server is on a public IP but the phones, Grandstream 2000s, are natted.

Here is my problem. When I plug in a phone I can make and receive calls fine. After about 5 minutes, however, the phones seem to “timeout” and calls roll to voicemail without the phone ever ringing. The link between my provider and the server seems to be fine as the calls route to voicemail. Also, I can pick up a handset and make a call and then that handset can recieve calls again for about 5 minutes. I have tried various locations for the phones…so I do not think it is anything that is related directly to one router or another. Any ideas as to what settings or changes I could make to fix the issue would be appreciated.


Art Sexton