The Nokia comes with a Generic SIP client. There are several SIP providers that have near free plans. (Basically, you get x00 free minutes every time you recharge your account. Since you have to recharge your account, it's not really free, but you only start using your credits after you used up your free minutes).

Here's one company, but there are many more (all run by the same corporating with different country lists. And they keep on changing them, so you got to be a little bit careful if the "free" country is still free):

http://www.voipbuster.com/en/index.html

Martin