I'm having the EXACT same problem. Calls to (outofstate) Calif, Ky, and (instate) Oregon drop after 14 minutes and a few seconds.
I, too, have a Netgear 6200 but have recently switched from comcast to DSL.
I don't have SPI on the Netgear. I have under SETUP/Wan/ NAT Filtering Secured/OPEN
and Disable SIP ALG. There also is "Disable Port scan & Dos protection"
Joe F