The exact amount is dependent on physical properties of the medium. That is, the signal doesn't distort uniformly. If every component decreased by the same amount, the signal would be weaker, but not distorted, and amplifying the signal would restore it. Because the received signal is distorted, however, amplification simply magnifies the distortion and probably won't help. The key is to receive enough of the signal so that the receiver can figure out what the original signal was.
Engineers tend to use bandwidth to refer to the spectrum of signals a channel carries.
Note that we can potentially achieve a higher data rate by switching the voltage faster. The encoding method determines the amount of information carried in one baud. Use 4 different voltage levels. It should be clear that sending data at 38400 baud over a voice grade line simply won't work. Even at 9600 baud only the first and second harmonic are transmitted, and the signal will be severely distorted.