I have my computer and my Amstrad em@iler on the same line and I've had no probs at all. I'm signed up with Wanadoo.
Both my computer and em@iler are connected to an ADSL filter which is connected via a 2 meter extension cord to the BT socket.
Perhaps you could try a shorter extension cord or another ADSL filter (Just in case there's a prob with the one your using)...
www.freewebs.com/dacvolvo