Also check that you haven't typed a comma instead of a full stop. And if it is all correct, you MAY find that on another day all is well - it's possible tht the ISP was having a bad day when you tried to connect.
You need to establish that you have the correct drivers for your modem and also that there are no conflicts with other hardware. You can probably obtain drivers on the net if you establish the model of modem
Check in device manager to see that modem is properly listed and has no warnings against it or conflicts
Ensure password and user name together with dial up number are all correct and then try again are correct
If all else fails you can buy a new internal modem with drivers cd for around £10. Uninstall the existing and drivers and start again. This may well be easier