UMA, that is Unlicensed Mobile Access is the commercial name for the 3GPP Generic Access Network (GAN) standard. In the UK Orange is the first operator to have the ratified version of GAN in operation. Previously BT led the way in 2005 with their own pre-GAN version called BT Fusion.

On the Blackberry 9700, for example, when using GAN the handset displays UMA in place of the 2G/3G symbol. Therein lies the clue as to what this is all about.

Clipped from Wikipedia UMA/GAN use is described thus;
The most common application of GAN is in a dual-mode handset service where subscribers can seamlessly roam and handover between wireless LANs and wide area networks using a GSM/Wi-Fi dual-mode mobile phone. GAN enables the convergence of mobile, fixed and Internet telephony, sometimes called Fixed Mobile Convergence.

The local network may be based on private unlicensed spectrum technologies like 802.11, while the wide network is alternatively GSM/GPRS or UMTS mobile services. On the cellular network, the mobile handset communicates over the air with a base station, through a base station controller, to servers in the core network of the carrier.

Under the GAN system, when the handset detects a wireless LAN, it establishes a secure IP connection through a gateway to a server called a GAN Controller (GANC) on the carrier's network. The GANC presents to the mobile core network as a standard cellular base station. The handset communicates with the GANC over the secure connection using existing GSM/UMTS protocols. Thus, when a mobile moves from a GSM to an 802.11 network, it appears to the core network as if it is simply on a different base station.
So, if you set up your Blackberry device to use your home WiFi network then it will automatically find the Orange GANC (Generic Access Network Controller) in their network and connect you to it to make and receive calls instead of using the GSM/UMTS airwaves. This all completely seamless to the user, there is nothing to actually do other than use the handset as normal.

Thus far Orange lead the way in Europe, it seems.
On September 25, 2006 Orange announced its "Unik service“ [6] [7]. The announcement, the largest to date, covers more than 60m of Orange’s mobile subscribers in the UK, France, Poland, Spain and the Netherlands.
Interestingly, UMA devices also include the emerging Femtocell - Vodafone in the UK are the first to offer a commercial product and we can expect others to follow.

And so within the UMA umbrella we have two distinct methods. There is the UMA-enabled handset as currently seen in a number of Blackberry handsets. And then there is the Femtocell that eliminates the need for a specific handset at all.

I do think that the latter is the way forward, and that it will make the need for UMA-enabled handsets obsolete and that will be the end of that particular line.

A useful reference is the UMA Today website. Here you can read a lot about the technology. Also you can see the latest list of UMA-enabled handsets, Femtocells, and Mobile Network Operators who carry the technology.

UMA seems quite an interesting technology, exciting even. But those with capped bandwidth on their ADSL connections should be wary. It can be too easy to use up your data cap if not careful.