There are many factors which can affect the quality of a Skype call, including the hardware setup at the callers end, the network infrastructure at both locations plus, the all important internet connection. The following documents will help narrow down the cause of the problem and ensure that the network infrastructure is configured to provide the best environment for Skype calls.
How much bandwidth does Skype need?
TalkShow Advanced Network Tips
This document details networking best practices and should help identify any problems on the network which maybe affecting the quality of the Skype call.
Microsoft’s Skype IT Administrators Guide
This goes into much more detail and is designed for system and network administrators.