When you think about it, every telephone line in your home may cause interferance on a DSL line. Not to mention use power on the line to power the telephone itself. I was getting a bit upset because I thought my ISP was jipping me by decreasing bandwidth when the speed test were showing my bandwidth dropping down between 300-450 Kbps. Soooo, I started by disconnecting one phone at a time in my home starting with the oldest. Older electronics usually are not as efficient as newer ones. Disconnecting the oldest phone instantly brought me up to approx. 850 Kbps. Hmmmmm, interesting! I disconnected all the phones and my speed returned to what I origianlly started with when I set up the DSL to 1.2 Mbps. Unfortunately, with DSL, many of us pick it because we can have the telephone and computer on the same line. So I had to hook up the telephones again. Minus the old one!!!!
Give it a try and see how it works. Just remember, not everything is the computers or ISP's fault.
Another tip from a different visitor:
Beyond disconnecting phones on DSL line, try running a seperate line right off the box. one other thing that I tried based on expensive phone cables out there, is exposing the wires and twisting them around each other as tight as possible. This gave me a 400kbps increase, give it a try it is awesome!