if it were just one of your phones, I would say it's the phone, but since it's al l three, it's one of two things, either local interference, or interference on the transmission cable. If there are any sources of EMF noise inside or around your house, such as an old microwave oven, cell repeater tower, electric blankets, electric heating pads, toasters, etc. When I am siting in my office, I can hear signals being sent to my cell phone over my office landline, and the old Nextell cell phones were horrible for this. I remember walking across an office, and watching computer screens get all fuzzy when I got near them because of my Nextell cell phone. You could try unplugging all of your electrical appliances and seeing if that helps. You can also go outside to your telephone interface and plug your phone in there to isolate your inside wiring.

The other source, on your transmission lines has to be referred to your telephone company. They will run tests on the line to check for bridge-taps, bad repeaters, and shorts in the cables that connect your phones to the central office switch. It sounds like you might be getting bleedover from a data ckt in the same cable on a different pair.

hope this helps!