NEW YORK ( TheStreet) -- What if I told you there was a device that strengthened cellphone signals so your phone worked reliably inside your house -- upstairs, downstairs and even in the basement?Interested? Well, what I told you, sorry, you can't have it no matter how much money you throw at the companies offering it? That's what is happening all over the country. The device is a tiny cell tower called a femtocell, named for the mathematical term for one-quadrillionth the size of a unit. It can improve indoor cell signals dramatically. Unlike signal boosters, which take and amplify the stronger outdoor signal indoors, femtocells work even with terrible outdoor signal. They use a home's broadband Internet and a GPS signal to communicate with the nearest mother tower. Indoors, these petite, portable mini towers broadcast a cellular signal -- not Wi-Fi -- of several thousand square feet so phones remain connected to a cellular network. But today, only customers of AT&T ( T), Sprint ( S) and Verizon ( VZ) can get one for about $100 to $300. One regional U.S. carrier, Mosaic Telecom in Wisconsin, also offers one for $50. The wireless carriers may toss one in for free, especially if you threaten to leave due to poor indoor reception. Everyone else? Too bad. T-Mobile, MetroPCS and most other carriers don't offer it. Even some customers of the ones that do such as Sprint-owned Virgin Wireless and Boost Mobile -- are out of luck. Granted, the big three make up about 80% of the U.S. wireless population but it's often the customers of smaller companies that could use the boost. "Coverage is the main reason why consumers churn and leave their existing operator," said Andy Germano, vice chairman of the Small Cell Forum, an organization promoting the development of these mini cell towers. "Giving someone a low-cost femtocell to address that issue is almost a no-brainer." The reason why femtocells are not available in stores like Best Buy ( BBY) or from every wireless carrier is the devices operate on licensed wireless spectrum, the same spectrum that companies like Verizon, AT&T and Sprint pay billions of dollars to the government. Each femtocell must be registered and approved by its cellphone carrier before it starts working for you.