There’s one exception — or, rather, 8,500 of them: T-Mobile’s archipelago of hot spots at Starbucks, Borders and other public places. In these places you encounter neither the fee nor the Web-page sign-in that you would encounter if you were using a laptop; the words “T-Mobile Hot Spot” simply appear at the top of your screen, and you can start making free calls.

The cool part is that, depending on how many calls you can make in hot spots, the Wi-Fi feature might permit you to choose a much less expensive calling plan. If you’re a heavy talker, you might switch, for example, from T-Mobile’s $100 plan (2,500 minutes) to its $40 plan (1,000 minutes). Even factoring in the $10 HotSpot @Home fee, you’d still save $600 a year.

SAVING NO. 2 T-Mobile’s billing system isn’t smart enough to notice handoffs between Wi-Fi and cellular networks. So each call is billed according to where it begins. You can start a call at home, get in your car, drive away and talk for free until the battery’s dead.

The opposite is also true, however; if you begin a call on T-Mobile’s cell network and later enter a Wi-Fi hot spot, the call continues to eat up minutes. If HotSpot @Home catches on, therefore, the airwaves will reverberate with people coming home and saying, “Hey, can I call you right back?”

SAVING NO. 3 When you’re in a hot spot, T-Mobile has no idea where you are in the world. You could be in Des Moines, Denmark or Djibouti. So this is a big one for travelers: When you’re in a hot spot overseas, all calls to United States numbers are free.

SAVING NO. 4 T-Mobile’s hope is that you’ll cancel your home phone line altogether. You’ll be all cellphone, all the time. And why not, since you’ll now get great cell reception at home and have only one phone number and voicemail? Ka-ching: there’s an additional $500 a year saved.