Here’s a brilliant idea: get your cell phone to call via Wi-Fi through the internet, for nuttin.

That’s what T-mobile announced last week at about the same time iPhone came on the market, like, so no one knew. But the New York Times was paying attention.

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.

These phones hand off your calls from Wi-Fi network to cell network seamlessly and automatically, without a single crackle or pop to punctuate the switch. As you walk out of a hot spot, fewer and fewer Wi-Fi signal bars appear on the screen, until — blink! — the T-Mobile network bars replace them. (The handoff as you move in the opposite direction, from the cell network into a hot spot, is also seamless, but takes slightly longer, about a minute.)

So, I’ thinking, when your browser slows to a crawl you’ll know this idea is catching on.