This is a problem that mobile phones’ manufacturers have to face because unfortunately not all aspects of computing technology develop according to Moore’s law, and one of these is the battery. Most mobile phones are powered by lithiumion batteries. These batteries are popular because they can offer many times the energy of other types of batteries in a fraction of the space. In the current state of the art, chemists cannot sufficiently increase the amount of energy created by the chemical reactions. At the moment the only way to create more powerful batteries is to make them larger. However this does not well match with the evolution of the mobile terminals which tend to have less room available for the battery in order to accommodate additional components and technologies. Many attempts have been made to extend the battery life without renouncing to use the services that 3G phones offer. For example authors in [5] show the benefit of cooperation among mobile devices willing to share their wireless resources. Cooperation allows to reduce the energy consumption of mobile devices and at the same time to increase the quality of some services, such as file streaming or file downloading, as well as web browsing. Moreover authors in [8] describe another efficient way of saving energy in mobile VoIP. Instead of having a data connection open with WLAN when waiting for an incoming VoIP call, they suggest to use the GSM network as a wake-up signalling channel when the connection to the SIP server is needed.