First step is to check that the telephone line is working properly and has a normal dial tone. When you plug a normal phone into the socket do you get a continuous dial tone with no breaks? Do you have any special services on that line, for example withholding your own number when you make a call so that the person at the other end can't see your number? Do you have any other services plugged into the t/p line via an extension such as Sky?
You know the number that is being called via the modem. Use that number on your telephone and listen to the result. You should hear some high pitched whistles and burbles indicating that the server at the far end has answered. If you don't get that, what do you get?
--------------------
Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.