Nice info. It answers the question of why voice quality goes down as the signal get weaker. If it is weak the voice sounds robotic.
A vocoder analyzes voice and transmits the characteristics of the voice. Something like teleporting a dog house by measuring it, determining the materials it is made of, and sending the detailed description over the radio. On the receiving end the detailed description is used to select raw materials, and assemble them to make a replica of the dog house.
P25 applies strong error correction to the important parts of the voice description, and lesser or no error correction to the details. So as the signal gets weaker the details are dropped due to errors and the voice sounds less natural.
P25 is one complicated protocol.