I have an Ooma telo, sitting behind my Netgear WNDR3700 router.
When I plug the Ooma into the router, my download/upload speeds go from 6.56MPBS/.73MBS to .95MPBS/.195MBS.
I can place and receive calls with no problem, but on line games slow to a crawl. As soon as I unplug the Ooma, the connection speeds go right back up.
Currently, I have the Ooma with a fixed IP of 10.0.0.100 (in range with my router's DHCP) and I have reserved that address in my router as a fixed IP. The QOS on the Ooma is set to 0 and the Oomas DHCP is set to 10.0.0.200 to 10.0.0.201.
My Netgear router is currently sitting in the DMZ zone of the Actiontech GT704Wg modem/router provided by Verizon (NAT is set to on for both routers). If I connect the PC to the back of the Actiontech router, and leave the Ooma on the Netgear, I get a full speed connection. If I move the Ooma to the back of the actiontech, the connection slows down.
Everything seems to work OK if I put the Ooma in between the Actiontech and the Netgear then let the Ooma do DHCP for the Netgear, but this won't work with the Xbox.
Try setting your Ooma device's QoS upload/download speeds, instead of to zero, to about three or four thousand MPBS more than your Internet provicers upload and download speeds.
Place your Ooma device's reserved IP address in your Netgear's DMZ. (My not help, but worth a try).