This page gives information about Radio Frequency Interference, the unwanted reception of radio signals. You'll find out how to determine what is at fault (the transmitter or the receiver) and what you can do in either case.
The US Federal Communications Commission's Compliance and Information Bureau has created an online electronic version of their "Interference to Home Electronic Entertainment Equipment Handbook." It's a bit heavy on the graphics for my taste, but you can hardly get more definitive. The executive summary: interference from licensed radio transmitters being defective is so incredibly rare that they do not even discuss it; instead they focus on ameliorating the technical flaws of the cheap, shoddy piece of consumer electronics you were so proud of getting at such a good price. They do, on the other hand, warn that that cheap, shoddy piece of computer electronics you own probably is to blame for generating TV interference, as opposed to your TV.
Radio transmitters do not transmit at audio frequencies. Therefore, radio transmitters are never at fault when a telephone receives a radio signal. You can get a basic article about telephone RFI, reprinted from QST (the magazine of the American Radio Relay League) and available from the ARRL Info Server, via the link above.
The Federal Communications Commission (FCC) has prepared a telephone interference information bulletin to assist you in understanding why interference to your telephone system occurs. (This bulletin, FO-10, is also available from the ARRL Info Server.)
The FCC also recently (1994) surveyed a selection of telephone instruments to determine the susceptability of different models to RFI, as well as the efficacy of various forms of remedy. (This report is available directly from the FCC, online, from ftp.fcc.gov.)
Finally, I have a copy of an old Bell System report on suppressing RFI, from back when failed telephones cost the manufacturer money, rather than the consumer. There's a lesson in there...