Something to try since you have your Ooma device behind your router:
If your router has a SIP setting, try disabling the SIP setting.
Another thing to try is:
You could setup your router to issue the same IP address to your Ooma device (Static IP Address).
Than port forward (or open ports in your router) UDP ports 10,000 to 20,000. These are the actual ports that carry voice traffic.
Note: There are several other ports that Ooma uses, and they might have to be opened also. But try these ports first.
If it is easier for you, Place your Ooma device in your router's DMZ. The Ooma device will behave as if it was connected before the router. (You need a static IP address for this configuration also.)
Something else that adds stability:
a. Access your Ooma Telo “home port” by plugging in a patch (network) cable from the Ooma Telo home port to a computer’s network card port. Sometimes you have to restart your computer after installing the patch cable, so that a proper connection occurs.
b. Type http://setup.ooma.com
in you browser address window and hit enter. The “Ooma Setup” window opens.
c. In the left hand Navigation window, select “Network”.
d. Under “Modem Port MAC Address”, select Use Built in:
e. Select the “Update button”.
f. Do a “cold boot” of the Router.
g. After the router cold boot, do a “cold boot” of the Ooma device.