Step 1. Check if driver has been installed properly:
Go to Start > [Settings] > Control Panel > System >[Hardware] > Device Manager. The modem should be seen in Device Manager without any exclamation marks or question marks under the Modems category.
If modem has exclamation mark or not listed there and there is an Unknown, Unsupported or Other Devices there - remove it from Device Manager and reinstall the modem.

Step 2. Check if your System can communicate with the modem:
In Windows XP/2000:
Go to Start > Control Panel > Phone and Modem Options >Modems > select your modem > Properties > Diagnostics. Click on Query Modem button.

In Windows 95/98/ME:
Go to Start > Settings > Control Panel > Modems >Diagnostics tab
You should see a new modem installed on one of the COM ports (eg.COM3).
Select the COM port with your modem, Click on 'More Info...'button.

After a while, you will see a window with the response from the modem.
If you are getting 'Port already opened' message, you need to disable software which may use the COM port of the modem (e.g.faxing software) and reboot computer. If you are getting an error about modem not responding, try to reinstall the modem's driver.

Step 3. Check if your dial-up connection is configured to use the right modem:

Windows XP:
Start > Control Panel > Network Connections. Right-click on the connection you use to dial and select Properties. Make sure under "Connect Using" you have got the right modem selected.

Windows 95/98/ME:
If you had another modem installed before, you need to reconfigure your Dial-Up connection to use the new modem. To do that please go to My Computer > Dial Up Networking > right-click on your ISP connection icon and select Properties. Under Connect Using... menu select the right modem.

Step 4. If modem does not detect the dial tone:
Make sure the phone line is working and connected to the LINE socket of the modem.

WINDOWS 95/98/ME: go to Start > Settings > Control Panel >Modems > select your modem > Properties > Connection >Advanced > Extra Settings...

WINDOWS XP/2000: go to Start > Control Panel > Phone and Modem Options > Modem > Properties > Advanced

In the Extra Settings field type one this string which disables the dial tone detection:

ATX0

Put comma before the telephone number that you are dialing (for example: "01912345" change it to ",01912345").