PPPoE Error 633 — What It Means & How to Fix It

What Does This Error Mean?

Error 633 means Windows thinks your modem is already being used by another program, or the modem is not correctly configured. It is like trying to make a phone call when the line is already busy. Usually a restart or a quick driver fix resolves it.

Affected Models

Windows XP
Windows Vista
Windows 7
Windows 10
USB modems
Any dial-up or PPPoE connection

Common Causes

Another application is already using the modem's COM port
A previous connection session did not terminate cleanly, locking the port
The modem driver is outdated or not installed correctly
Windows Telephony service (Tapisvr.exe) is stuck or not running
A USB modem was unplugged without properly disconnecting first

How to Fix It

Restart your computer. This clears any program that has locked the modem's port.

This fixes Error 633 in the majority of cases.
Open Device Manager, find your modem, right-click it, and select Uninstall Device. Reboot your PC. Windows will reinstall the driver.

Reinstalling the driver resets the modem's COM port assignment.
Open Services (type 'services.msc' in the Start menu). Find Telephony, right-click it, and select Restart.

The Telephony service manages modem COM ports. Restarting it frees locked ports.
If using a USB modem: unplug it, wait 15 seconds, and plug it into a different USB port on your computer.

USB modems sometimes lock themselves to a specific port. Switching ports forces a fresh start.
Check if any fax software, virtual COM port tool, or remote access program is running and using the modem. Close those programs and try again.

Programs like fax applications often grab the modem COM port and forget to release it.

When to Call a Professional

Error 633 is almost always fixable without calling your ISP. If you have tried every step and still see it, contact your modem manufacturer's support. This error is Windows and driver related, not an ISP network issue.

Frequently Asked Questions

Is Error 633 a sign that my modem is failing?

No, it is almost never a hardware fault. Error 633 is a port-locking issue caused by software or improper disconnection. If the error goes away after a restart, your modem is fine.

Why does Error 633 keep coming back every time I restart?

If it comes back repeatedly, a startup program is likely claiming the modem's COM port. Check your startup programs (Task Manager > Startup tab) for fax or remote access software. Disabling those programs should stop the error from recurring.

Can Error 633 happen on a broadband (not dial-up) connection?

Yes. Although it originated as a dial-up error, Windows also shows Error 633 on USB broadband dongles and some PPPoE configurations. The fix is the same — release the locked port and restart the telephony service.