Right, I could so solid-fluid revision or answer you guys...
Skype is THE best voice program, I have tried 3-4 now. The features are numerous ,plug in a headset and pay nothing to get calls from PC to PC anywhere in the world for free. Excellent quality***. Upgrade to skype out (I believe the call cards you can buy are 10€), fantastic call quality for about 1p/min - brilliant if you talk to people in foreign places. Ok, 1p may not be amazing compared to some house phone deals but for people like me with no landline anymore, it's fantastic. In addition to that it makes no difference whether I call UK, US, Poland etc.
Finally there's skypeIN. Skype in costs about £1.50 or something per month. They will give you a physical phone number (like I have a manchester phone number picked from a list). When somebody calls it gets router to your computer, if your computer is off it can be re-routed to your mobile or voicemail.
***When I say voice quality is excellent, it is 90% of the time. However, on occasion I don't get equal (or better) to traditional telephone quality and there's a delay and sometimes broken up. In the last month this has NEVER occured on skypeOUT (i.e. paid for calls) but only on the PC-PC calls. I also have an awful connection so I believe this is the actual cause.
Anybody with a PC for communication should use it, I know I do twice as much as MSN these days.