I did a Google for error 50 dial-up, and found a forum on this very subject. It may have to do with a third party ISP like AT&T, etc. Look at Google's list I believe it was the 4th or 5th listed that had the same question as you.
I?m trying to use my laptop (windows 2000) on dial up after a long bout with faster internet. It dials correctly but then gives me the message (6) that the handle is invalid. Now, it has moved on to error 50 ? the network request is not supported. What should I do?