In Building Bi-Directional Cellular Amplifiers work by taking the outside signal, amplifying it the re broadcasting it indoors.
Systems like this (such as those used in vehicles and buildings) can adversely affect your service providers network. This can occur when the inside and outside antennas are too close together and signal oscillation occurs. It's like feed between a microphone and amplifier. Other users in the area can be knocked off their phone calls. This does not make cellular service providers happy.
Bi-directional amplifier manufacturers know this can occur so they usually build in some type of warning system such as having lights go on or off or change color. The drawback to this solution is that if no one is looking at the amplifier, (which is usually hidden away under a car seat, closet or attic) the warning light(s) are not noticed.
Wilson Electronics came out with a better solution they call SmartTechnology. Their SmartTechnology controls the power of the amplifier to a service providers tower and makes sure it is within the carrier’s specifications. If too much power exists so that oscillation occurs, their bi-directional amplifiers will shut down. Their SmartTechnology amplifiers will also reset automatically when the condition no longer exists.