I think you might be talking about AT&T 3G microcell: http://www.wireless.att.com/learn/why/3gmicrocell/
I don't think making one of these on your own will be very easy.
I don't think what you're trying to do it possible, setting up your own microcell-to-VoIP.
If it were, people would be setting these up everywhere for free cell phone service. Unfortunately there is licensing on the radio frequencies and you can't just set up your own phone cells. Companies pay a lot for the rights to those frequencies and will not allow you to use them for free.
On the other hand, you can use smartphones and SIP service to get a VoIP system going over WiFi.
The smartphone idea is one we're thinking should be implemented either way, it's inexpensive enough.
I thought low-power cellular devices were allowed as long as they were within a certain mw and range? More researching I suppose.
Thanks everyone or your help thus f ar.
Ahh I finally found something almost exactly what I was looking for:
http://openbts.sourceforge.net/
http://www.networkworld.com/news/2010/083010-open-source-voip-cell-phones-at-burning-man.html
Pretty much the exact thing we were looking to do, except someone else has done it, so it's no longer allowed. Oh well.