A Simple But Effective AC line RF Filter.
If your telephone or answering machine is powered by the ac power line, just filtering the telephone line may not be enough. Sometimes the interference comes in through the ac power cord. In this case, you may need to filter the telephone line and the power line.
A simple but very effective ac line RFI filter can be constructed using a $12.00 ferrite toroid (model FT240-43, from Amidon Associates in Santa Ana, CA (phone 714 850-4660) and winding 20 to 25 turns of the ac supply cord wire onto the toroid.

The "filter" installed on a cordless telephone base unit.

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MFJ Enterprises also makes an ac line filter (model MFJ-1164).

RF Filter