More on Hotspot @Home
It is a service based on Unlicensed Mobile Access (UMA) technology. So it allows users to seamlessly switch voice calls and data connections between a Wi-Fi and cellular connection, depending on which is available and most cost effective at a particular moment.

For example, a user can be on a cellular-call, using the minutes from their mobile plan, but when they walk into their home or in range of a T-Mobile hotspot, the call is automatically moved over to Wi-Fi without interruption, saving those and possibly improving reception. The same would happen while streaming audio over T-Mobile's EDGE network on the data side, for instance.

T-Mobile HotSpot @Home costs $20 a month on top of your regular cell phone plan. It delivers unlimited voice over Wi-Fi calls from T-Mobile's more than 8,000 hotspots and your home through a Wi-Fi access point. You pay $5 more per month for each additional family member. And you may also choose to have the service deduct minutes from your regular bucket of minutes in your cell phone plan.