said by bobjohnson :

T-Mo hotspot service is not femtocell it's just your phone using wi-fi with voip... femtocell is like a miniature cell tower using your internet connection as a backhaul line so you can use it with any GSM phone.

said by KrK :

That's why I said "like T-mobiles." It's not quite the same tech, but it's the exactly the same idea. IE reliable Cell service in your home, going out on your broadband. I like this idea, provided AT&T doesn't still try and charge you for minutes. If the calls go out on your own internet, then there should be no minute charges to your plan.

Now, if they DO use your minutes, then they shouldn't charge you anything for the hardware. It should be free.