So I did find the problem with my air flow , it was my fault, I put in a hepa filter as a replacement, I learned not to do that again, ford factory replacement installed, problem goneI've wondered the same thing. The system overall just doesn't seem to have much air volume from the fan to blow hard through all of the vent registers.
It's as @ccw Said that the fan speed drops to avoid noise so call quality isn't impacted.Not sure this is an issue, but it was a little weird. I assume it has something to do with power draw, but I also thought with all the batteries and heavy duty alternator it would be a non-issue.
It was 36 degrees outside.
Truck was running in park and the engine temp was normal range.
HVAC was on max fan speed (7) and "HI" temp (no defrost options going).
Heated front seats were "on" hi.
When I went to make a phone call with my iPhone (manually) while connected to CARPLAY the fan speed for the HVAC dropped to what seemed like a 5-6.
I thought it was a coincidence so I tried again and again - same. As soon as I terminated the call the fan speed went back up. There was no visual notification of the fan speed change, I only was able to tell by sound.
Are my batteries going?