Degrade

Low-quality digital sampling

Headroom Peak clipping threshold
Quant Bit depth (typically 8 or below for "telephone" quality)
Rate Sample Rate (turn control to left or right for different sound characters)
Post Filter Low-pass filter to muffle the distortion produced by the previous two controls
Non-Linearity Additional harmonic distortion "thickening" (turn control to left or right for different sound characters)
Output Level trim