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The microfilters that most service providers issue are designed to filter out higher frequencies, thereby blocking the ADSL frequencies and not the voice frequencies. According to the instructor, quite a few of the cordless devices have the same sort of filters built into them to cut down on the interference that they place on the line. Good quality phones actually do a pretty good job. But some of the filters from cheap manufactures may actually have the reverse effect on your ADSL.
He demonstrated this by connected a cheap cordless phone directly to the back of a Cisco 677, and it immediately dropped train. After removing the cordless phone it trained right back up.
I would highly recommend that you connect your cordless phones in another area of the house to decrease the chance that it may cause problems.
In fact, any device that connects to the phone line (alarm systems, satellite dishes, game console systems, faxes and modems) have the potential to cause interference. If you suspect that there may be a problem, remove the device and see if your connection improves.
Also read About DSL for lots more information