First determine that it is not a inside wiring or equipment issue. Unplug the telephone cord from the L1/L2 jack on the eMTA telephony modem and plug in a known good wired phone and give it a good test. if it still does it, then Comcast will need to come out to replace the eMTA.
If it works with a phone connected directly to the eMTA, then you have telephone equipment, or inside wiring / jack issues that need addressing.
