Fix Error 633: Modem or Other Connecting Device Already in Use

Error 633 (“The modem (or other connecting device) is already in use”) occurs when Windows tries to establish a dial-up or VPN connection but the modem or virtual adapter is already occupied by another process. The fix is to close the program holding the device, restart the relevant Windows services, or reboot the machine. Below is a detailed guide to resolve it permanently, with background on why it happens and how to prevent it.

What Causes Error 633

Error 633 is a Windows networking error that appears in the system event log and in the connection dialog when you attempt to start a dial-up connection (PPP) or a VPN connection that uses a virtual modem. The operating system treats the modem or virtual adapter as a single-owner device: only one process can use it at a time. When a previous connection did not close cleanly, or when another application (like a fax program, a remote access client, or even a stuck background process) grabs the device, the next connection attempt fails with 633.

Common triggers include:

• An interrupted dial-up or VPN session (e.g., computer slept or network cable was pulled).
• A third-party dialer or legacy fax software that leaves the modem port open.
• Windows Telephony (TapiSrv) or Remote Access Connection Manager (RasMan) service stopped or hung.
• Multiple network adapters or virtual modems with conflicting configurations.
• Outdated or corrupted modem drivers.

Quick Fixes to Try First

Start with the simplest steps, as they resolve most cases:

1. Reboot the computer. A restart releases all handles on the modem and restarts the telephony services. This clears the error in about 70% of cases.
2. Close any program that might use the modem. Look for fax software, remote desktop tools, or legacy dialers in the system tray. Right-click and exit them fully.
3. Disconnect and reconnect the USB modem (if external). Unplug the device, wait 10 seconds, and plug it back in. For internal modems, use Device Manager to disable and re-enable the device.
4. Check for a stuck process. Press Ctrl+Shift+Esc to open Task Manager, look for processes named rasphone.exe, dialer.exe, or msimn.exe (Outlook Express), and end them if present.

Restart the Windows Telephony Services

If a reboot is not possible or the error persists, manually restart the two services that manage dial-up connections:

1. Press Win+R, type services.msc, and press Enter.
2. Find Remote Access Connection Manager (RasMan). Right-click and choose Restart.
3. Find Telephony (TapiSrv). Right-click and choose Restart.
4. If either service is not running, set its startup type to Automatic and start it.
5. Try the connection again.

Note: On some systems, the Telephony service depends on the Plug and Play service. If it fails to start, check that Plug and Play is running.

Update or Reinstall the Modem Driver

Outdated or corrupted drivers are a common underlying cause. To update:

1. Right-click the Start button and select Device Manager.
2. Expand Modems (or Network adapters for a virtual modem).
3. Right-click your modem and choose Update driver.
4. Select Search automatically for drivers. If Windows finds none, visit the manufacturer’s website for the latest driver.
5. If the problem started after a recent update, use Roll Back Driver to revert.

For a virtual modem (like the one used by Windows VPN), you can also remove the device from Device Manager and then scan for hardware changes to reinstall it automatically.

Check for Conflicting Network Adapters

Multiple active network adapters can confuse the connection manager. If you have both a physical modem and a virtual modem (e.g., from a VPN client), disable the one you are not using:

1. Open Control Panel > Network and Sharing Center > Change adapter settings.
2. Right-click the adapter you do not need and choose Disable.
3. Try the connection again.

Also, check for legacy dial-up adapters in Device Manager under Network adapters. If you see an old “WAN Miniport” or “RAS” adapter that is not in use, disable it.

Use the Windows Network Diagnostics

Windows has a built-in troubleshooter that can reset the network stack and clear stale connections:

1. Go to Settings > Network & Internet > Status.
2. Click Network troubleshooter.
3. Follow the prompts. It may ask you to select the connection; choose the one that fails.
4. If it suggests resetting the network, do so. This will reinstall network adapters and reset settings.

Alternatively, you can run the following command in an elevated Command Prompt to reset Winsock and the TCP/IP stack:

netsh winsock reset
netsh int ip reset

Then reboot.

Frequently Asked Questions

Why do I get error 633 even though I am not using the modem?

Windows may still think the device is in use because a previous connection did not release the handle. This often happens after a crash or sleep/wake cycle. Restarting the Telephony service or rebooting clears the stale state.

Does error 633 affect VPN connections?

Yes. Windows VPN connections use a virtual modem (WAN Miniport). If another VPN session is active, or if the virtual adapter is stuck, you get 633. Close all VPN clients and restart the RasMan service.

Can I fix error 633 without rebooting?

Yes. End the process holding the modem (check Task Manager), restart the Telephony and Remote Access Connection Manager services, or disable/enable the modem in Device Manager. These often work without a reboot.

Will uninstalling the modem driver help?

Sometimes. If the driver is corrupted, uninstall it from Device Manager (check “Delete the driver software”), then reboot. Windows will reinstall a fresh driver. This is a more aggressive step, so try it after other fixes fail.

Is error 633 a sign of malware?

Not typically. It is a standard Windows networking error. However, if you see unknown processes in Task Manager that use the modem, run a security scan as a precaution.

Why does error 633 appear on a fresh Windows install?

This can happen if the modem driver is not properly installed or if the Telephony services are set to manual. Ensure the services are set to Automatic and the driver is the latest from the manufacturer.

Preventing Error 633 in the Future

To minimize recurrence:

• Always close VPN clients or dialers before disconnecting the network.
• Avoid putting the computer to sleep during an active dial-up or VPN session.
• Keep modem drivers updated.
• If you use a USB modem, use a dedicated USB port and avoid hubs.
• Set the Telephony and Remote Access Connection Manager services to Automatic (they usually are by default).

If the error persists after all steps, consider that the modem hardware may be failing. Test the modem on another computer or replace it. For virtual modems, a clean reinstall of the VPN client often resolves the issue.

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