[phrider]

The Wall Street Journal has an article today on the relatively new femtocells by AT&T, VZW, and Sprint. It fails mention the long established T-Mobile UMA WiFi calling.

Business News & Financial News - The Wall Street Journal - WSJ.com "A Cell Tower of Your Very Own" and also supposedly on page D3 of today's print addition.

[MidnightDraven]

I would say its because a femtocell boosts carrier signal; and uma is calling over wi-fi bypassing the carrier network.
Different technologies.

[NJBlackBerry]

That plus the fact that some of us rarely use UMA - even where available - as the call failure rate is so high....