Xfinity Landline Phone will not ring if plugged into house phone lines but will if plugged directly

If the phone line out of the router is plugged into the house phone line and a house landline phone in another room is plugged into the same house line, the phone will not ring, but works otherwise (e.g. call can be answered). If the house landline phone is plugged directly into the Xfinity router, the phone rings when calls are received. This just started happening for no known reason.