Everyone is converging towards mobility. Heavy mobile users can save great amounts of money using VoIP services to make and receive mobile calls. The amount of money you can save depends on your mobile communication needs and habits and on the prerequisites of the voip services you use.
It is possible to make completely free calls from a mobile phone or portable device, provided you meet the following requirements. First, your phone or handheld device needs to be supported by the service you are using; second, your caller or callee needs to be using the same service; and third, your phone or handheld device needs to have an Internet connection. A typical scenario where you can make totally free mobile calls would be where you use a high-end device (e.g. a Wi-Fi or 3G phone, BlackBerry etc.) to call a friend who uses the same service on their mobile phone or PC, while you are in a Wi-Fi hotspot. That call would be free even if your friend is on the other side of the planet. Examples of such services are Yeigo and Fring.
That is quite restrictive and not everyone can live such a scenario or something similar. Not everyone has a sophisticated-enough mobile device, and not everyone has an Internet connection on their mobile (i.e. a data plan). But when calls are not free, they can be very cheap, with rates starting from two cents per minute on international calls. The services available have different features and ways of working - some use strictly the Internet backbone while other start calls on the GSM network and eventually route them through traditional phone lines and the Internet. Here are some links for getting started with mobile VoIP services from www.cheap-international-callingcard.com .