Just to provide some ideas on how to troubleshoot your issue I'll relate my own experience and hopefully it will help.
I once changed out my rental gateway with another one and after the activation my voice service would not work. The Line 1 light on the front of the gateway was lit up and that is the jack in the back of the gateway that I had the phone plugged into. On a whim I plugged the phone into the Line 2 jack on the back of the gateway and voice service worked just fine. So you might want to try plugging your phone into the other line jack as that could fix the issue.
If neither the line 1 or line 2 lights on the front of the gateway are lit up, then I would think you would need to contact Comcast and ask them about reprovisioning the gateway and activating the voice service.
Hope this helps you out.