1. Go into my computer > dial up networking > right click on dialer > goto properties.
2. check correct modem is selected in connect using. If incorrect click on drop down list and pick correct modem.

3. click on OK and try connecting again.
4. If that fails go into control panel > modems > check to see if correct modem is installed. if not remove modem and re-install.
5. If that does not work try creating a new dialer.