Femtocell devices extend and enhance cellphones coverage.

Most cellphone users have one time or another experienced with spotty coverage or simply dropped calls regardless of which carrier they get their service from. Over the years, there have been a number of devices intended to help with the problems by extending or enhancing cellphone service. In the past year, some of the biggest wireless carriers in the U.S. have begun offering new devices called femtocells that are small boxes that act like miniature cellular towers to improve the network's coverage. While Verizon Wireless and Sprint Nextel sell femtocells, AT&T does not. But here is a catch:the wireless carriers who do sell femtocells, do not market these devices as doing so would give a hint to a sensitive subject they rather do not want to talk about, coverage problem. So the catch is that if a subscriber really feel like he or she needs one, he or she will have to expressly demand for it. And usually carriers use the option to offer it as a last resort to retain subscribers who threaten to switch carriers.
According to the Wall Street Journal, "femtocells don't require special cellphones as they use the same radio spectrum as carrier's cellular towers to extend their cellular coverage by taping into a landline connection and routing calls over the Internet."
One femtocell can handle up to three simultaneous calls, covers up to 5,000 square feet and the owner can grant access to other cellphone owners that use the same carrier.
Sprint femtocell device called Airave is offered at $100 plus a monthly fee of $5, while Verizon unit called a Network Extender, goes for $250.
AT&T is said to be testing it's version of femtocell in three different cities without any mention of possible roll over date.

For more, see WSJ.com