Usually the modem pool (the bank of modems) at the ISP may be all busy and can't connect you. Other times, I would get that message when the ISP was down completely.
Whenever this happened, I would pick up the phone and manually dial the ISP dial-in number to actually hear what went on, either handshaking tones, or a busy signal... sometimes I would even hear the pre-recorded "We're sorry. All circuits are busy now. Try your call again later." message.
Tom