It depends on where the problem is. The most basic thing is of course to check your phones to make sure they are behind DSL filters (or are all filtered behind a Whole House DSL/POTS Splitter, which is found near your NID if one exists there). I would also check the condition of your phone jacks, making sure there is no corrosion or shorting cables/contacts. Lastly, if you can post up your modem Transceiver Statistics, we can see if anything stands out in there as well. If you don't know how to get these, tell us which modem you have.
