Unlicensed Mobile Access enables access to cellular mobile voice and data services over unlicensed spectrum technologies such as Bluetooth or Wi-Fi (802.11).
The idea behind this technology is the following: when there is an accessible Wi-Fi hotspot near you, the UMA-enabled device can connect to it and use the broadband connection for making and accepting calls and sending and receiving data. It is very similar to using wireless VoIP-telephony on your GSM mobile phone.
The UMA-enabled phone is also be able to use regular GSM base stations as any normal mobile phone can. The handset is able to seamlessly change connections between the licensed cellular radio access network and the unlicensed IP network, even in the middle of a call.
All you need to use the new technology is a UMA-enabled device, an operator that supports UMA, and an Internet broadband connection that you can access via Wi-Fi (WLAN).
The most important difference from the widely known VoIP technology is that UMA is tightly linked to the mobile radio network, which is used for routing, authentication and billing. A call initiated using the Wi-Fi interface reaches the 2G core network through the UMA Network and once the signal is transferred, it becomes indistinguishable from the rest of the cellular traffic.