In truth, it never left them: New Android-based phones from T-Mobile will include unlicensed mobile access (UMA) calling that allows talking over Wi-Fi or cellular networks without using special apps or VoIP as such. This is a change in tactics for the firm, which deprecated UMA for the last year or more. The service isn't yet available, nor was pricing discussed in the press release.
T-Mobile introduced converged Wi-Fi/cell calling using UMA four years ago; I wrote one of the first articles about this for The New York Times as it was launched in the Seattle area. The service slowly rolled out nationally, and, as far as I could tell, was a hit among the sweet spot of the audience. That was people who had poor coverage in the home, rather than those exceeding their cellular data pool.
Unlimited cell plans started percolating out a couple of years ago, and T-Mobile's offer there trumped any advantage from the flat-rate, unmetered Wi-Fi calling service. (UMA's other advantage is seamless handoff between Wi-Fi and cell during a call, also not an issue with unlimited calling.)
At some point in the last year, the company's UMA details started to disappear, and new phones weren't featuring UMA. As far as I recollect, only a few BlackBerry models could be purchased new with UMA, although existing converged calling customers could use the service without a change. And T-Mobile pushed the service for businesses, where UMA could integrate right into the enterprise's Wi-Fi network, providing better pricing and call quality than use of a plain cell plan.
Today's announcement puts UMA back front and center, although I have a hard time understanding why it's important to the company. It gives them a slight advantage in very narrow areas, especially for budget callers.
The general tech media is covering this as an innovation and something spectacular and new. It's the problem with short memories.