Most commonly but not always the telephone interference happens because the telephone wiring in the house wall and attic acts as a huge antenna which funnels RF into the telephone . In this case, this filter (which attenuates RF by a factor of 150) should completely solve your problem.The phone company usually doesn’t want to get involved in inside house wiring problems nowadays so they probably will not be of much help in this situation.
Another common scenario is when the telephone line that comes down from the pole to the house entry box acts as an antenna which funnels RF into the house wiring and ultimately into the telephone. In this case again the K-Y filter should do the trick. The phone company should also be able to come to your rescue in this situation by installing bypass capacitors in the entry box (there is a nice handy ground connection in the box for this) and/or installing an inductor.
However if the RF is being rectified (ie. changed from radio to audiofrequency) somewhere in the house or external wiring, an RF filter at the phone will not help at all.
RF can be rectified by corroded connections or even by other phones or answering machines connected to the system. For this reason it is imperative that you disconnect all other phones answering machines, etc. in the house before attempting to cure one
phone at a time.
If the filter reduces the interference but doesn’t completely get rid of it, two filters plugged in series will surely do the trick. If there is no change in the interference then either the RF is getting into the telephone through some other route (the handset cord , or a.c. line cord) or it is being rectified somewhere else in the local system.
A late ham friend of mine once lived in a single story house with a tri-band groundplane vertical mounted on his roof. He had horrible
telephone interference. It persisted even when the telephone was completely disconnected from the phone line because the RF was so strong and the telephone was very sensitive ( some phones are extra sensitive to RF). I talked him into trying a
dipole located further from the house and this brought the problem
down to manageable proportions.
Sometimes the telephone users body picks up enough RF to be capacitively coupled into the telephone by the persons hand. Holding the telephone in a different place helps in this situation.
Some telephones are so sensitive that if the length of wire between the filter and the phone is over a few feet long it will pick up enough RF to cause interference. In this case the short pigtail should be plugged into the telephone and a longer cord
should be connected from the filter to the modular wall jack.