This type of service also resembles residential VoIP services, but with an interesting difference - no monthly bills. You buy a device and install it at home or in your office, and you make and receive calls 'ever after' (so to say) without paying anything. At the time I am writing this, there are very few services like that. There is a trade-off between initial cost on one side, and call costs and restrictions on the other side.
Again, this kind of service is beneficial mostly to users in the US and Canada. There is no clear geographical restriction as such, but since the existing services are based and centered in the US, using this kind of service outside the US and Canada involves difficulties that somewhat cancel the cost savings.
Here is a brief presentation of the different existing services. ooma (yes, it starts with a small o) sells its hardware (a hub and a phone) for a relatively high price and allows you to make unlimited US/Canada calls for free 'ever after' (take this 'ever after' with a grain of salt). PhoneGnome works in a similar way, with some slight differences, namely in price and features. MagicJack sells a small USB device for bread-and-butter cheap, and allows free local calls afterwards, but requires a computer to make and receive calls. Finally, 1ButtonToWifi focuses on international calls and mobility, making them free or very cheap.
Finally, the 'no monthly bill' concept, while being true in many circumstances, is not completely translated in reality in all cases. You need to incur some costs every now and then, depending on how you use the service, e.g. making international calls, renewing subscription, getting additional features etc. Read more on these services: