Authorize Skype for a port different from 80 and 443

Hi all :)

Here's the situation: we have an ISA Server 2006, due to the policy of our company, we are blocking internet connections for all users (HTTP and HTTPS are blocked). Some users though require Skype for their work. since ports 80 and 443 are blocked, we fixed another port on the skype clients, we created a rule to authorize outbound and inbound trafic for that port for both TCP and UDP protocols. despite of that, skype is not working, the authentication fails with a message "unable to connect to server"

any ideas?

thank you all