What Causes Error 633?

Indications
An error message is displayed that reads either: 'Error 633: The modem is not installed or configured for Dial-Up Networking. To check your modem configuration, double-click the Modems icon in Control Panel' or 'Error 633 - the modem (or other connecting device) is already in use or is not configured properly.' Windows will usually list the device as installed and working properly, but the error message is displayed when the modem is used to dial out.
Possible Causes
A missing or damaged Telephon.ini file, or an absent or corrupted Telephony Application Programming Interface service provider registry entry can cause error 633, according to Microsoft. The error also can result from users attempting to establish more than one simultaneous broadband connection with a single modem, or other applications like fax or voice software using the modem. Error 633 is also caused by outdated software, conflicting with Windows XP modem drivers. Out-of-date Palm Pilot software, for example, is a frequent cause of 633 errors.
Solution: Repair missing/corrupted files and registry entries
To resolve a 633 error, first check to ensure no other applications are using the modem. Press 'Ctrl,' 'Alt' and 'Delete' to enter the Task Manager and scan the list of running programs. Terminate any that might be using your modem. Find out if the Telephon.ini file is missing or damaged by clicking the 'Start' menu, selecting 'Find,' and checking the box labeled 'Files' or 'Folders.' Enter 'telephon.ini' into the Search field and click 'Find.' If the file is present, rename it to 'telephon.old.' Click the 'Start' menu, select 'Run,' type in 'tapiini.exe' and hit 'Enter.' Restart the computer.If the error message persists, use the registry editor to determine the status of the TAPI service provider entry key. Click the 'Start' menu, select 'Run,' type 'regedit' and press 'Enter.' Navigate to the following registry key:HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\ Telephony\ProvidersFind the ProviderFilename0 value and change it to 'TSP3216L.TSP'. Save the changes, exit the editor and restart the computer.
Solution: Remove Conflicting Software
If the error is being caused by outdated software conflicting with the modem, it can be resolved by removing the conflicting software. Restart the computer and press the 'F8' key as it boots. Once the Windows Advanced Options Menu is opened, scroll down to the entry titled 'Safe Mode' and press 'Enter.' Once Windows loads, click the 'Start' menu and select 'Control Panel.' Click 'Uninstall a program,' select the entry for the conflicting software and choose 'Uninstall.' Restart the computer.