You are referring to what is known as a load coil. It is basically an inductor, not an amplifier. The most common type is the H88, meaning it has 88 mH of inductance. The first one is placed 3000 ft from the telephone office and then they are placed every 6000 ft after that. Their purpose is to combat the loss of high frequencies in the voice band by adding inductance to cancel the capacitive effects of long cable runs. A phone line will work just fine without load coils up to around 18,000 ft out though. If you have DSL then the load coils have been removed from your line. We do use a product known as "smart coils" if you have DSL and are more than 18,000 ft from the DSLAM.