Archived — Cutting Through…Interference on a Telephone

The main problem affecting telephones is audio rectification.

Do you sometimes feel like you're not alone on your telephone line?

Do you hear unwanted voices or sounds?

This disruption can affect the telephone or various other electronic instruments in the home. Sometimes, when a piece of equipment is surrounded by an intense radio signal, it can act as a radio receiver. This phenomenon, known as audio rectification, is caused by the telephone, and such equipment must be modified or repaired.

If your telephone is picking up radio signals, check to see if there is a broadcast or two-way radio station transmitter antenna in the area. Inform the station operator about the problem. He/she will likely cooperate to help you eliminate the interference.

NOTE! A more expensive telephone is not necessarily any more effective. A telephone with several options, such as hands-free mode, memory, call display and a built-in answering machine, has many electronic components that can make it more susceptible to interference. If the telephone set is rented, the telephone company should help you to eliminate the interference.


This procedure can help you find the potential source of interference.

A. Disconnect all telephone equipment or all accessories connected to the telephone, such as answering machines, modems or fax machines. Proceeding by elimination, reconnect one telephone or accessory at a time and check whether the interference is still present. If you have only one telephone, borrow another one. Telephone companies also have telephone sets that are protected from radio frequency interference. If the interference persists on this type of equipment, the line is likely the cause. The telephone company should then be notified. Go to step D. If, however, the interference affects only one set, that set is the problem. Go to step B.

B. The interference may affect components in the telephone, primarily:

  • the telephone cord;
  • the handset cord; or
  • the telephone case.

1) Replace all long cords with shorter ones.

2) Wind the excess cord around your hand and cover the resulting coil with adhesive tape. This makes a good homemade filter.

3) If the interference persists, it may be necessary to install filters at each end of the telephone cord or add ferrite cores. These devices help block out interference and allow only telephone communications to pass through the line. They are sold in some stores specializing in electronic parts.

4) You can also try a telephone set with better protection against interference. However, the interference may also be produced by telephone accessories. So before rushing out to buy these filters, go to step C.

NOTE! If filters remain the only solution, it may be less expensive to replace the set than to add filters. Consult a technician or a supplier.

C. If the interference is coming from a telephone accessory, the process of elimination must again be used. The cords and connections on various accessories may be faulty. If the problem persists after all these checks, go to step D.

D. There is a good chance the problem is in the telephone line outside your home. Find out if your neighbours are experiencing the same problem. If so, consult with your telephone company which may install filters or shielding as the ultimate solution.


Cordless telephones, which continue to grow in popularity, are actually radio transmitters/receivers that are subject to interference. In addition to picking up unwanted radio signals, the cordless telephone may also pick up transmissions from other cordless telephones, baby monitors or portable intercoms using the same frequency.


  • Change channels if possible. On some models, this can be done simply by pressing a button.
  • If not, move to another room.
  • Or, if audio rectification is the problem, install filters or ferrite cores.

For more information on filters, refer to the brochure: Cutting Through… Various Solutions to Interference.

Cutting Through… Interference on a Telephone
(PDF, 212 KB, 7 pages)

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