Skype is a voice comms program that works through most firewalls and the quality is excellent. The basic program is free and allows you to make PC to PC calls anywhere in the world for free.
If you pay a small charge you can make PC to Pots (plain old telephone system) calls at much lower rates that the traditional telcos can offer, this facility is called SkypeOut. And now they have launched, wait for it, SkypeIn. For €30 per year, approx £20 you can buy a telephone number that anyone can ring you on. Sadly the UK numbers sold out virtually straight away.
This really is a great product that is going from strength to strength. They now have a central contacts list which means you don't have to copy it from PC to PC plus there are versions for Mac, Linux and it's a killer app for the Pocket PC.
If you want, you can Skype Me!