900MHz phones, in particular Panasonic phones have been reported to cause problems. The base of the phone will broadcast a signal to the handset to initiate the ring, even before you pick it up. It's possible that this is an issue. Completely unplug the cordless base from the jack and have someone call to see if that helps.
It could also be a voltage drop across the phone wires when a ring signal is sent or a phone extension is picked up.

FAQ by kadar