I've been noticing that my 3Mbit/768Kbit DSL line (verizon) has been constantly dropping and going back up. A while back I was getting more answering machine messages at home so I moved my answering machine from the main floor of the house to my basement office. The phone port I put the machine on also has a dsl modem and another phone on it.
So I used a Y splitter to send one side to the dsl modem and the other to the dsl filter, answering machine and phone.
/ dsl modem -- lan
phone jack | ---(splitter)/
\ dsl filter -- ans machine -- phone
Sometime after that all this flaky DSL behavior started. So on a whim I unplugged the dsl filter/answering machine/phone. At this point the dsl modem was the only thing on the line. At this point it got more stable.
Is there an electrical reason for this, like is it bad to have a DSL filter plugged into the same jack as used by the modem? Note that the filter is not upstream of the modem (in that case it wouldn't work at all) but just nearby. I'm thinking there might be some kind of reflection off the filter that's adding noise to the line? My other phones then might not be a problem because they and their associated filters are upstairs and far enough away to not cause a problem.