Windows XP, 2000, 98: Open the Control Panel and double click Internet Options. Click the Connections tab, highlight your Internet connection and click the Settings button. Click the Advanced button in the Dialup Settings section. Check the option Disconnect when connection may no longer be needed.
Windows ME: Open the Control Panel and double click Dial-up Networking. Right click your Internet connection and choose Properties. Click the Dialing tab. Check the option Disconnect when connection may no longer be needed.
To manually disconnect your connection in Windows: Right click or double click the connection icon in the System Tray (It will normally look like two little monitors that occasionally blink), and choose the option to disconnect. If you have a shortcut on your desktop to your Internet connection, double clicking it will also give you an option to disconnect.
Macintosh OS 8-9: Unfortunately, there is no option to be prompted to disconnect when Internet applications are closed. There are two ways to manually disconnect. Click the Apple menu and choose Remote Access Status. This box shows you the current status of Remote Access; if you are connected, it will give you an option to disconnect. You can also open the Remote Access Control Panel and choose disconnect there as well.
If you have properly disconnected the modem, but the computer is still connected and the modem still has control of the phone line, it is possible that an error has locked the modem. You may need to power cycle the computer to resolve this.
It is also possible that the modem is damaged. If your phone line is taken off-hook by the modem anytime it or the computer (if it is internal) is powered on, it is "beyond repair". Symptoms would include only being able to make the call if you plug the phone cord into the modem when you're ready to dial, or your line going 'dead' as long as the modem is plugged in. This is fairly common after a thunderstorm especially if the phone line is not surge protected. Contact the modem or computer manufacturer for further support.