I know it's happened to me before, and it's probably happened to you, too. You're talking just fine on your cell until you get inside an office building or a high-rise of some type, possibly riding up in an elevator, and there goes the signal. There's no doubt coverage is better these days than it used to be. Wireless carriers have added more than 30,000 new towers and cell sites in just the last two years alone. However, this is still physics we're dealing with here, and there are still trouble spots. The New York Times recently ran a story that focused on a new technology called femtocells that at least one carrier is currently deploying to improve coverage in small areas, such as your home. The article went on to mention the use of cell phone repeaters, or boosters, as another possible remedy, saying..

"The presence of boosters on frequencies that belong to the carriers has made them somewhat controversial. The Federal Communications Commission certifies the devices for some purposes, but they recommend contacting your carrier before installing one.

If a person is interested in improving coverage with a booster, “we would encourage that person to first work with his or her wireless carrier to identify a certified device that the carrier also approves of before installing it and using it on the network,” said Robert Kenny, a spokesman for the F.C.C."
  
It's not only important for someone interested in installing a booster to contact their carrier, it's actually against federal law to do so without your carrier's permission. The Telecommunications Act clearly states that a licensee must approve use of any transmitter in its spectrum, and the reason for that is clear. Improperly installed, a booster might do wonders for your coverage, but wreak absolute havoc with your neighbors'. That could disrupt such important transmissions as emergency calls to 911, and that's one of the primary reasons why protecting the integrity of the network, everyone's network, is important. Using repeaters or boosters on frequencies licensed by the government, without your carriers' consent, means you are violating the terms of the license.  So before you take matters into your own hands, make sure you're not doing it at someone else's expense.