Three phones on our system have developed a buzzing when using the intercom. If the phone is called and is on handsfree, the caller hears a loud buzz. If an intercom call is made from the phone, there is a loud buzz. If the called phone is picked up, the buzz goes away. If the phone is moved to a different jack, the buzz stays with the phone, not with the jack. Replacing the RJ11 cord does not fix the problem.

Any suggestons?