Next troubleshooting step is to unplug the phone cord on the eMTA telephony modem and plug in a known good wired phone directly to the L1/L2 jack. Give it a test.
It the issue goes away by bypassing your inside phone wiring, then the issue is with the inside phone wiring, jacks and splices.
If the issue still happens connected directly to the eMTA, then the next step would be contact Comcast support and they can do a health check on the eMTA and decide if a tech needs to come out or not.
