Originally Posted by glupa4e
...I have never configured DMZ or opened any ports. I am with the default settings of a Slackware64 14 full install.
Fair enough. Well, a down-and-dirty way to test whether it's a port issue would be to put your machine in a dmz. Be *warned* that this effectively removes your box from your router's default firewall. Just follow your router's documentation in order to do this; it should be a fairly trivial process.
My google-foo indicates that skype generally uses ports 80 and 443, but since it's proprietary and relies on proprietary mim servers, it's a toss up. According to this:
it appears that your call quality may improve if you dedicate at least one port (other than the default) to listening, and open that port on your router's firewall if your machine isn't dmz'ed.
I don't have the greatest internet bandwidth speed, and the few times I've used skype it's done its job, so I can't serve as a decent example, but I would expect that unless your speed is horrible, you should be getting decent results by default.
Hope I haven't muddied the waters.