Odd problem - the PC is fairly old, with an external modem, and was working (i.e. connecting to the internet via dial-up) fine. Now though, with no apparent changes in the interim, whenever the lead is plugged into the phone socket (even when the PC is switched off) the dial tone disappears - there is no sound on the line. Therefore, if a connection is attempted it says 'No dial tone', although once the lead is removed from the socket the dial tone reappears. Anyone had similar problems?
How do you check for the dialtone on the line when the modem is connected? Have you got a 2 into one socket with a phone attached?
Maybe its to do with the REN. There is a theoretical maximum number of apparatus you can plug into a domestic telephone system I think its four. Are you exceeding this - If so unplug another phone extension & try again.
Thanks for the ideas everyone, I shall try experimenting. Taff36 - I check for a dial tone on an extension in another room; also, I have not added any more phones in the house since the problem occurred, so I don't think it can be this, although I shall try unplugging the others just to be sure.