The latest generation of Facebook Messenger, M4, started rolling out in October 2018. It was a massive effort that took several hundred people and more than a year to launch; I was one of about 20 designers to work on it full time.
M4 aimed to be the most streamlined Messenger ever released, and my job was to radically simplify the audio and video call interface. I collaborated with a few other designers to design a system of all the states in a call (ringing, connected, missed, dropped, etc.) and then I documented the specs in a component library available to every designer within Facebook.