Always start CDV troubleshooting by resetting the eMTA telephony modem. There is a small indented reset button on the panel where the cables attach, you will need a toothpick or some kind of poker.
2nd step is to disconnect the inside phone wiring from the eMTA's L1/L2 jack and plug in a known good wired phone directly to the eMTA and test it. That will isolate if the issue is with your inside telephone wiring or not.
With the phone connected directly to the eMTA, call Comcast support and they can do a health check on your eMTA to determine if the eMTA is getting good signal levels and history of poor signal issues.
