That your filters are on all lines save that DSL modem. Next disconnect all the phones except a non-wireless phone and see if it fixes it. If so, add one phone at a time until you find which phone is creating an issue.

Another owner found a defective DSL filter so if dropping back to just one phone doesn't fix it, swap that phone and filter for another.

Read http://forums.cnet.com/5208-7589-0.html?forumID=62&threadID=245356&messageID=2473098# as well.

Bob