The answer is in your question, FaceTime has to do with a face-to-face call and whenever you lock your phone. It’s telling the system to shut off temporarily ‘causing it to think that you’re not going to be in the call (FaceTime).
A: It’s not unusual to have a dropped FaceTime call, and there are several possible causes. The first thing to check is whether you and the person you are calling both have strong Wi-Fi signals (the weaker the signal, the less data-carrying capacity.) Then scroll down and make sure “Wi-Fi Assist” is on.