Skype makes use of Peer to Peer technology which is ordinarily banned. An exception however has been made if Skype is configured to work correctly on the College network.
To correctly configure Skype please go to:
Tools-Options-Connections and uncheck "Use port 80 or 443 for incoming connections". You should also specify 4123 in the "Use port 4123 for incoming connections"
Tools-Options-Advanced uncheck "Start Skype when I start Windows"
and make sure "Check for updates automatically" is checked so you are running the most recent version as there have been a number of security vulnerabilities with Skype in the past.
The use of Skype is deprecated as it uses non-standard voice protocols and call quality can be an issue. Alternatives such as Sipgate are much better and offer a SkypeIn style local UK number at no cost unlike Skype.