Modem reports "Error 50" on attempt to connect

I receive an "Error 50" when I attempt to connect to my internet provider. I have checked the modem and it is working properly. I have reinstalled TCP/IP services, etc.

The modem dials, connects to the ISP provider, the ISP verifies my user name and password, and then I get the Error 50 message and am unable to connect.

I have tried 3 different modems (all good) with the same result. I have rechecked the settings numerous times, set up new connections, etc., and end up with the same result.

Any ideas?

Thanks.

 

Is this connected to a DSL line?

 

This is a regular dial up connection. The PC was upgraded from Win98 to Windows XP Home Edition. The modem drivers were upgraded to Windows XP drivers. That isn't the problem.