T-Mobile USA quietly launched Unlicensed Mobile Access (UMA) services this week, becoming the first US carrier to allow calling over both wifi and cellular networks.

The operator has launched a site – theonlyphoneyouneed.com – which is offering UMA handsets to users in the Western Washington/Seattle area on a trial basis.

UMA technology allows dual-mode phones – those with both cellular and wifi radios – to route calls via wireless hotspots where available. To access to the HotSpot @Home service users need to buy a UMA-capable handset for $49.99 (£27) with a two year rate plan costing $39.99 a month.

Details on the website are scant but the phone in the image looks to be a Samsung. Subscribers will also receive a wifi router which plugs into their broadband connection at home. Calls are then made over wifi when at home or at a T-Mobile hotspot and over cellular when outside of wifi range.

Telecoms.com got wind of this trial earlier this month and believes T-Mobile is rolling out UMA in a bid to maintain a competitive edge while it rolls out 3G.

Work on its 3G network has already begun, with a majority of the rollout to be completed before 2008. But in the meantime T-Mobile plans to fill its speed gap by sending calls over other carriers’ broadband networks.

Most interest in UMA technology has come from carriers with both fixed and mobile networks with a view to Fixed Mobile Convergence (FMC) strategies.

But with the exception of a handful of countries in Eastern Europe, T-Mobile’s parent, Deutsche Telekom, has no fixed networks outside Germany. However, Robert Dotson, chief executive of T-Mobile USA, recently said that UMA was important to the operator because more than 10 per cent of T-Mobile USA users do not have a traditional home phone.

By selling, or even giving, T-Mobile USA subscribers wifi hubs to install at home, T-Mobile could try to hitch a lift on DSL and cable broadband deployment in the US.

With UMA, calls made over a competitor’s fixed line network would generate billable Call Detail Records (CDRs) on T-Mobile’s own network.