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.
Our OS who art in CPU, UNIX be thy name. Thy programs run, thy syscalls done, In kernel as it is in user!