re: firewall traversal

It may be that Skype is not doing anything extraordinarily new, but from my limited experience, it is the only VOIP client that works through the firewall we have at work, which is very restrictive - ie, no UDP, TCP only to 80,443 and 8080.
I guess Skype just tries every possible solution, where none of the other clients go so far, even if it actually doesn't seem that hard to do.