Here's the basic idea. If you're willing to pay $10 a month on top of a regular T-Mobile voice plan, you get a special cellphone. When you're out and about, it works like any other phone; calls eat up your monthly minutes as usual.
But when it's in a Wi-Fi wireless Internet hot spot, this phone offers a huge bargain: all your calls are free. You use it and dial it the same as always — you still get call hold, caller ID, three-way calling and all the other features — but now your voice is carried by the Internet rather than the cellular airwaves.
If you think the system is working, ask someone who's waiting for a prompt.