Telephone interference is sometimes more of a problem for the station than it is for neighbors. It is made of plastic and of course there is no shielding and the RF is just slam-dunking on it. It seems to help. He uses rf shielding extensively throughout the plant. The floor, the ceiling and the walls are all screen mesh copper.
Telephone wiring is distributed in conduit separate from station audio, control and power wiring.
That's the point where you eliminate it. You use filters up close to the phone. If that doesn't do the trick, I look at the spring cord. With the AM, I don't think I've had any problem I can recall caused by the spring cord. Ten Tips on Solving Telephone Interference 1.