Over The Air reported in the past that T-Mobile was running a trial for a service whereby cell phones that have Wi-Fi on board can roam seamlessly between Wi-Fi hotspots and cellular networks. Looks like the trials were successful, because T-Mobile decided to deploy the service across the entire U.S. starting in mid-June. Who will benefit most, businesses or consumers?Before we answer that question, let's take a quick look at the details. When the service launches next month, it will be limited to only a handful of Nokia and Samsung handsets. Customers will pay $20 above and beyond their existing plan fees and will then be able to use their home Wi-Fi network or any of T-Mobile's 8,000 hotspots spread across the United States. Subscribers will have to pay an additional $5 per month to add more lines to the service. T-Mobile will allow people to use their existing Wi-Fi routers at home, but also is going to offer a free (after rebate) router of its own that will improve device battery life, among other things.

The benefit here is that once phones switch to a Wi-Fi network for calls, they will no longer be using their monthly minutes. In addition, the Wi-Fi hotspots can provide coverage in the home or office environment where typical cellular signals may be weak.

At first glance, the service seems to be aimed at consumers. I'd argue that businesses, however, will be the early adopters here. Frank Hanzlik, managing director of the Wi-Fi Alliance, told The Wall Street Journal that because people spend so much time in their offices, businesses and not consumers will represent the largest potential customer base. "I think that enterprises are going to drive this trend aggressively," Hanzlik said. "That's where the low hanging fruit is."

Other carriers in the U.S., namely SprintNextel and AT&T (Cingular), have also conducted fixed-mobile convergence trials. Neither has announced any intent to offer the service nationwide.