A simpler way to help reduce the voice volume problem would be a speaker volume control that is linked to the microphone average volume. If the person is talking loudly, make the phone loud as well. Most people use the received volume as feedback for their own volume. When the other person is shouting at them, they either shout back (if angry) or quiet down. This is not fully reliable, but as an unconscious response it is something that people will not forget or need training.