Do artigo da Wikipedia sobre Femtocell :
In telecommunications, a
femtocell—originally known as an
Access Point Base Station—is a small
cellular base station, typically
designed for use in a home or small
business. It connects to the service
provider’s network via broadband (such
as DSL or cable); current designs
typically support 2 to 4 active mobile
phones in a residential setting, and 8
to 16 active mobile phones in
enterprise settings. A femtocell
allows service providers to extend
service coverage indoors, especially
where access would otherwise be
limited or unavailable. The femtocell
incorporates the functionality of a
typical base station but extends it to
allow a simpler, self contained
deployment; an example is a UMTS
femtocell containing a Node B, RNC and
in some cases GPRS Support Node (SGSN)
with Ethernet for backhaul. Although
much attention is focused on WCDMA,
the concept is applicable to all
standards, including GSM, CDMA2000,
TD-SCDMA, WiMAX and LTE solutions.
Há mais uma tonelada de informações nesse artigo. Não vou repetir tudo aqui, mas existem várias discussões sobre "lub sobre IP" e VoIP com adaptadores de terminal, etc.