Skype uses tcp-ip hole punching, basically trying to initiate a connection on both ends. guessing exit ports and trying to convince the firewall that this it re

Skype uses tcp-ip hole punching, basically trying to initiate a connection on both ends. guessing exit ports and trying to convince the firewall that this it received a response from the other side so it should start routing traffic to that port back. The solution is not full proof and takes a few attempts to predict the outgoing ports.