Roaming between mobile networks and fixed wireless networks using UMA technology is interesting for fixed-line operators because it enables them to bill for calls originating on unlicensed spectrums of the fixed-wireless network.

The original specifications were developed by the UMA group, which included suppliers such as Nokia, Ericsson, Motorola, Nortel, and Siemens. Ericsson has run UMA trials with several operators, including TeliaSonora, Vodafone and British Telecom. BT has already launched its BluePhone product and BT Fusion service while several other operators are planning on launching UMA services. Ian Livingston, CEO of BT Retail, said in The Register: "We know that many of our customers enjoy the convenience of their mobile phones when they're out and about but switch to using a landline phone when they arrive back home to save money or because they have little or no mobile coverage." The specifications for UMA are now covered by the 3GPP and will be included in their next release, version six.

UMA is not actually a product but a standard which allows for the handover of mobile voice and data to a wireless local area network (such as WiFi or Bluetooth). It works by allowing a mobile subscriber with a UMA-enabled, dual-mode handset, to connect to an unlicensed wireless network when he/she moves within a certain range. A UMA network controller then authenticates the handset and allows access to GSM or GPRS services on the fixed-wireless network.

A more complete convergence of fixed and mobile networks will be achieved when IMS (IP Multimedia Subsystem) is fully rolled out in the networks. This will introduce IP as the universal protocol that all services will run on. But, because many operators are still focused on delivering voice over circuit-switched networks, this level of convergence is still a long way off. Therefore, UMA functions successfully as an initial trial for fixed-mobile convergence.