Incoming audio and video calls from Facebook Messenger will look the same to iOS 10 users as incoming phone calls.
Vice president of messaging products David Marcus said in a Facebook post (embedded below):
Starting this week with iOS 10, receiving a Messenger audio or video call will feel exactly the same as a normal call, even when your phone is locked. This is a major improvement for all of you using Messenger on iPhones. We hope you will like it as much as we do! Readers: What do you think"

...