Many issues for the Comcast Phone service can be rectified by resetting the Cable/Telephony Modem. On the Arris models the reset button is a little indentation next to the ethernet port on the back of the cable modem. Can be depressed using the tip of a pencil or paper clip. Sometimes a missing dial tone can be restored by depressing the reset button for ten seconds and then allowing the lights to come back in sequence on the modem. Remember, however, if you have the Comcast Digital Voice Service and you are on that phone it will drop your phone connection. Is not always the answer but is worth a shot sometimes.
Especially if you are attaching the device (modem) to different devices i.e. routers, laptops. If one has internet services running through this modem the reset button has to be reset to connect to the new laptop, router, etcetera.
If one has number porting or provisioning issues this will probably not help but can help with some minor phone issues.