The telephone system in my home is a hodgepodge of wiring and assorted equipment. The house is vintage 1971, and has had some additional extension wiring added (some by me) over the years. The equipment collection is varied: a Radio Shack cordless phone (in the hamshack), a GE combination answering machine and telephone, an old GTE desk phone, and a couple of Bell South slimline phones, and a few modems.
The RFI symptoms I have depend on the transmission mode. When I talk on 20M SSB, the result is a muffled, unintelligible speech that interferes with the conversation. When I operate 20M CW, the result is a loud screech whenever the key is down.
I went through the usual exercise of disconnecting all but one phone, checking for any phones that were sensitive to the RFI. All of them were -- in all locations.
I decided to try a product I had seen advertised in QST (and carried by HRO, AES, etc.). It is the K-COM RF-1 filter for telephones. I called K-COM and ordered one. After some consultation with the lady who takes orders, I ordered the HF model, which filters 3-30 MHz. The filter arrived in a few days by first class mail, and I plugged it in and tried it. The RFI was gone! On CW, my test-person (XYL) had to strain to hear anything at all. Nothing heard on SSB. Maybe my problem was solved.
I called K-COM and ordered some more filters. They had my call sign, and used that to quickly find my ordering information. The reorder also came quickly, and I installed two more of the filters. Mostly they work (I still get RFI in the answering machine/phone, and something's amiss in the wiring on a wall phone). Along with the product is packed a two-page instruction sheet that covers different ways phone systems are wired, common problems (grounds, for instance), and suggested placements of the different K-COM products. The instructions are very well written and understandable.
I'm satisfied with the product - good quality and good service. A similar product can be obtained at the AT&T Phone Center Store at Baybrook Mall, but it costs a bit more and doesn't have the high-quality information sheets.
I still have some RFI, but I'm gradually hunting it down and cornering it.