The service lets users of a special mobile phone make calls over Wi-Fi from their homes as well as T-Mobile hotspots. 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, says 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 answer they most commonly gave was that the phone had to work better in their home, he says.

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 $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.