picocell (Score 1)

You are describing a standalone GSM picocell e.g.
http://www.ipaccess.com/ipaccess_2004_pages/bts.ht ml
http://www.rivanetworks.com/nano/nano.htm

To operate one legaly you would presumably need to get an FCC experimental license for limited power output, or permission from an existing carrier.

One problem with most picocells or microcells is that they are designed to talk to the rest of a GSM network (billing and provisioning systems) and cant really be attached to a simple POTS or PRI line.

A personal GSM/VoIP gateway is one of those problems that has to get solved eventually though...