The phone cable is not plugged in.

The phone line really doesn't have a dial tone.

The phone cable is faulty.

The phone cable is plugged into the wrong spot in the modem.

The phone cable is plugged into a network adapter and not the modem.

You may have a telephone service set up on your phone line which gives an initial dial tone that the modem does not recognise. Try adding 3 commas to the start of the dial up number. This will put a pause before the modem begins to check for a dial tone.

Voice mail is available through the phone company and there are messages waiting. If this is the case then either turn off wait for dial tone in the Control Panel / Modem Properties or use the init string ATX3  in extra settings on modem properties to turn off detecting a dial tone.

You are going through an office telephone system.

The modem is broken

Another program or piece of software could be causing it. Shut down the computer, restart and try again. If it is still the same then press CTRL-ALT-DELETE, end task all programs except EXPLORER and SYSTRAY and try connecting again.

The modem could be just having a bad day, lets face it, we all have them. Try turning off the modems detection of a dialtone and see if it works. You just never know......