These pictures are generally too big to show in line, so view each one as you read each section.
These are a few DTMF numbers generated by computer in the telephone 8bit 8kHz sampling format. Note the noise columns when there is signal and the absolute silence (black) after the numbers stop. 8bit 8kHz audio is by no means a clean format.
This is what a voice recording looks like in telephone audio format. Notice the telephone has trouble with middle frequencies and that the noise floor is always high.
This file came from one of my linux voice modem programs I tested. Notice the bias is low, the waveform is asymmetrically smashed, and it was recorded really low from the large blocks in the area that is supposed to be silence.