Why does the heaphone audio input make poor quality calls?

[AC Question]

I have a really nice set of gaming headphones. (HyperX Cloud) When I connect the headphones to my phone and make calls, the quality is poor.

To test this, I call myself and leave a voicemail. When I use no headset and talk into the phone regularly, the call is clear; when I interchange a cheap set of SkullCandy earbuds (during the same call) the call quality gets bad (distant and scratchy); and if I interchange my nice HyperX headphones, they sound better than earbuds, but my voice is still tinny and compressed sounding.

You'd think that my headphones just have lousy hardware and don't record audio very well...

But, if I go to the video recording app on the same phone, with the same headphone/mic setup, and record a video: the Audio that is recorded through the microphone is robust and detailed. I can hear the sound of cars driving outside. I can hear the resonant booming bass of the human voice. The audio recorded from the headphone's mic is BETTER than the audio recorded with the default in-phone microphone.

So what gives? Why is my audio input crap on phone calls but not for other phone functions?