I haven't tested it on dialup yet
I don't believe that they have that functionality working yet. They are planning on that, but at the moment, it's only pc2pc.
I'm looking at changing though after doing some system monitoring I noticed skype constantly writes to the registry and sends,receive something over the net but I'm yet to find out what. I have also noticed some memory issues.
Don't know what it's writing to the registry, but the developers say that the stuff going out over the net is one of two things. First, the client needs to communicate and let others know that you are connected, otherwise people couldn't call you. Secondly, I believe that they use non-NAT'd nodes to help route traffic from NAT'd nodes. It's p2p software, and I believe that the p2p side of things is this routing of data through these other computers. That said, the signals are end to end encrypted, so it doesn't really matter if it goes through some middlemen.
Skype didn't work for me (probably a NAT problem on my end) - but maybe that was a good thing!
That's interesting. It seems to have no problems going through my setup which (unfortunately) goes through 2 NATs. (Yuck, but I have a DSL modem/router and a Netgear FVS318 switch. I need to have the switch... and I haven't gone out and found a cheap DSL modem sans router)
Here's the skype forum:
http://forum.skype.com/bb/