There are multiple kinds of devices. Femtocell is basically a little cell tower that is in your home/business. UMA is when you can do voice over WiFi and it was backed by T-Mobile, but they have shied away from it and not introduced it with Android. It's somewhat tricky and needs phone support (for the handoffs between networks), whereas any phone can support femtocell.
Re:DEF CON 19 - hackers get hacked
by drzaiusapelord on Wednesday August 10, 2011 @12:32PM
(#2868989)
Voice over WIFI is doable with Android. The latest update to my Vibrant installed the feature.
No argument here, I'm not an expert... but isn't this exactly what the microcell devices that AT&T (etc) are giving to customers do? You set it up on your home broadband and nearby users invisibly use that "tower" and it goes over your home network.