Mobile telephony works on GSM networks, which, in a way, cover the whole world. VoIP has to use IP networks like WiFi, 3G, WiMax, GPRS, EDGE etc. This is the main obstacle barring the way to seamless VoIP telephony. Yes, if you want to make VoIP calls using your mobile you need to find a wireless way of connecting to the Internet. A data plan for mobile phones is very often required, if you are under a WiFi hotspot.
For using mobile VoIP, you need to have a mobile phone that is compatible with the service you choose, download and install a softphone, and use it to make and receive free calls to and from your buddies who have the same service. Other companies have pioneered in saddling VoIP over GSM itself, and in embedding VoIP functionality into a SIM card.
You could call through VoIP on your mobile phone just like you would on a PC, but through a wireless network. So, basically it can be as free as with software based VoIP service, as long as you call people using the same service on their PC or mobile phone. But your cost will not always be nil since you need connection through a wireless service - a data plan (which is not free, in most places). WiFi can be free in hotspots or at home - if you have a wireless router showering your house with Internet connection from your ADSL line. But then still, you need to have a mobile phone that has WiFi connectivity.
Anyone that has a need for mobile telephony (who hasn't?), and who wants to cut down the cost of this expensive way of communication. It can prove to be more profitable for heavy mobile users like business people and travelers.
