Much cheaper and easier options
Skype is way cheaper than any landline based VOIP service to probably all destinations, and it can be used with Fring on your cellphone.
Even better and cheaper is VOIPRAIDER. They offer 90 Free days of calls provided you top up initially with $10 credit. These free days of calls are to 'free destinations' include SA, USA, UK, AUS, NZ and all of the EU. I have been using it almost daily to call NZ for the last 3 months and hasn't cost a cent....
Even better is the fact that you can connect landline to landline for about 7c US = +- 50c. And if the 2 countries are 'free' destinations, e.g. SA and NZ then you don't pay for the call itself.
Once the free days are up... costs are about half that of skype.
And you can set it up as a SIP service on FRING on your cell phone as well. So no need for a computer. Only thing about this is that you need a 3G connection to get a decent call quality.