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, says 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. On June 18, Cincinnati Bell Inc. launched a similar UMA service in its region. Operators including Orange SA in Europe have also launched the converged services.