How do you hang up a call with AirPod pro?

Just press one of the Airpods once on their stems quickly. The "pressing" is just a forced pushing together of the stem that feels like a button-press but there is no real "button". That will accept a call, end a call, play music, or pause music.

You will feel a kind of sensation like you are actually pressing a button but it's actually a clever haptic feedback vibration trick that Apple did there.