Free calls already available on US femtos
Operators including AT&T and Sprint are offering free calls on their femtocells as an option already. They have existing ways to show the end user that the phone is using the femtocell (and therefore the call qualifies for the cheap / free rate), but the UX-Zone app would do this nicely as well (there - admitted it! :-).
Andy Tiller, ip.access (one of the companies involved in UX-Zone).