T-Mobile USA is launching a converged mobile phone service which lets users with dual-mode phones make cheap calls on their home Wi-Fi and at T-Mobile hotspots.

After trials in Seattle, the service will be launched across the US, making it very likely the world's biggest converged service. Other converged services include BT Fusion and France Telecom's Unik.

Users can also make regular cell phone calls when out of range of a hotspot. Calls hand off between the two networks if a user moves from their home, for example, beyond the range of their Wi-Fi network.

T-Mobile has been working on this service for around four years, said Mike Selman, T-Mobile director of marketing. The company began the project after asking customers what would make them give up their landline phones and rely only on their mobile. The most common answer was that the phone had to work better in their home, he said.

Because cellular networks often don't work as well indoors as outside, T-Mobile is using Wi-Fi to provide better service to customers in their homes.

For now, customers can choose from a Nokia or Samsung phone. Each costs US$50 with a two-year contract. Customers can use any access point, but T-Mobile encourages them to use either the D-Link or Linksys routers it offers. The access points, which are free after a mail-in rebate, come loaded with software that gives voice calls priority, helping ensure better quality.

T-Mobile customers must have a cellular plan of $39.99 or more in order to sign up, and then must pay an additional $19.99 per month for unlimited local and long-distance calls from the hotspots. A family plan that includes up to five lines costs $29.99 per month.

T-Mobile is also offering an introductory rate that doesn't expire. Customers who sign up while the offer is valid will pay $9.99 for one line and $19.99 for up to five.

Users can also opt to have their Wi-Fi minutes count the same as their cellular minutes under a regular plan, and not pay the additional monthly fee.

If a cellular and Wi-Fi network are both in range, the phones default to the lower-cost Wi-Fi network. Also, when users move from one network to the next, the call is billed based on which network started the call. For example, if a user begins a call on the cellular network and then walks into their home, even if the call is transferred to the Wi-Fi network, the entire call will count against their cellular voice plan.

Users can make phone calls from any of T-Mobile's 8,500 hotspots across the country, located in Starbucks coffee shops and other locations. The phone automatically connects and authenticates at the T-Mobile hotspots.

In many cases, customers can also use other hotspots. If the hotspot has no security, a user can connect to it and make calls. However, if a hotspot employs a browser redirect page that requires a user to agree to terms, the user won't be able to connect. That's because the available phones can't display HTML Web pages.

Users can also type in a password to connect to a secured Wi-Fi network.

Thomas Hagan, a T-Mobile customer in Midway, Georgia, who tried the service for four weeks, said he had no trouble using the phone in hotels, a local bookstore and his corporate Wi-Fi network.

He found it very easy to set up the router in his home and connect to the phone to it. Overall, he was impressed with the call quality over the Wi-Fi network and didn't have any problems with calls dropping when transferred between the cellular and Wi-Fi networks.

"The phone was a little larger than my existing phone, so that took a little getting used to," Hagan said, although he added that he ultimately took to the phone. Customers can only choose from two devices and they're both low-end flip phones with limited functionality.

T-Mobile relies on technology called Unlicensed Mobile Access (UMA) to offer the service. Cincinnati Bell has launched a similar UMA service in its region. Orange's Unik and BT's Fusion are also based on UMA.