* You are dialing the wrong number
* There is a problem at your ISP or in the telephone network
* You have voice-mail or other custom calling feature that produces a stutter dial-tone
* You haven't specified the proper country settings for your modem and the number you are dialing, or your modem doesn't support the country you are in.
* If you include the disable call waiting code in your dialing, and your phone line does not support call waiting; or, if you have call waiting, but your phone company doesn't support the disable call waiting code.

http://www.modemsite.com/56k/duns676.asp