Alright, here's the deal with VOIP for those not in the know. I will use skype as an example because it's all I use, though I know that yahoo and others offer similar services.
It is most useful for outgoing calls, because you can sign in with a username and call other people also signed in to the same service (on their computers or phones) for free.
You can ALSO pay to call landlines, which is something quite reasonable in US/Europe, approximately 2 cents per minute with skype ("SkypeOut").
However, to have people call you while you're signed onto such a service, you have to pay a monthly fee of sorts to have a real phone number that diverts to your skype account when called ("SkypeIn").
I'm in the UK now and on a 6-month contract with 3's "skypephone", which I've been loving - they allow only skype-to-skype voip (in addition to your usual voice minutes), but it's over their 3G network, which is extremely handy for keeping in touch with family/friends in other countries who can sign on to their computers to chat. Especially because it allows one to circumvent timezone differences, as you can be signed on to skype all day long. Of course, the reason for getting the 6-month contract was so that it would run out just in time for the new edition of the iPhone

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