Visiting a friend in Switzerland (let's not talk about the fun I had getting there - at least, not yet!), he sold me on using Skype - aka VoIP.
Upon my return, I signed up at the Skype website
It's brilliant! Even though the website suggests a minimum of W2K or XP (it does run on Mac's and Linux machines as well), it runs well enough on my W98 system.
Sound quality is good and, if you have broadband connection, calls to other Skype users are free! The rates to non-Skype users are highly competitive too.
I know, this is starting to sound like a plug - but I can't (at the moment) recommend it highly enough...