Skype: The Definitive Guide

Skype displays any number of error codes and error messages if and when there is a problem making a call or processing a payment. These messages typically are not very user friendly; neither are they consistent across operating systems.

Skype Call Failure: Error 6503, 6504, 10500, or 10503

If you receive error code 6503, 6504, 10500, or 10503 when making a call, it is because the Skype network is temporarily overloaded. Wait a minute, and dial again.