If you're on an extension, then temporarily relocate to a socket off that extension. For example if you're currently upstairs, you'll usually have another phone socket for downstairs. Have you tried plugging in there?
Depending on how the phone system works in your country, you may be able to disconnect the entire extension to upstairs, if it exists and if its anywher near a decent telephone setup (as laid down by the phone company), may expose a hidden socket for you to play with and try.
Failing this, if there's a socket at the point in your house where the phone comes in from the exchange, try plugging into there, thus bypassing all internal wiring.
If your sockets are too inconvienant to try this setup, it may be worth borrowing a wireless router and card from somebody so you can plug the rourter in downstairs or wherever, and thus do tests and source wirelessly AND eliminate the chance of you having a second faulty USB modem at the same time.
The idea of this sort of troubleshooting is to find out where the fault is occuring. If the issues clear themselves up when you plug into another socket or the "root" of the phone service where it comes into your house, then you know its an internal problem and to call them out would cost you. The onus is on you to try and prove there's a fault on the network and you do this by working backwards along your phone setup until the next physical connection is off your property.
It also doesn't hurt to change the filters as well, just to rule them out.
And of course, if there is a fault in your wiring/with your phone cable, wiping the system won't make any difference at all.
Edited by Neil Jones, 25 November 2005 - 06:12 PM.