If you’re in the middle of a call, the last thing you want is for Facetime to shut down. Unfortunately, this can be a common issue. There are a few different things you can try if this continues to happen to you.
First, you can close and reopen the app. You can close an app via the app manager screen, by either pressing the home button twice on older iPhones or swiping up from the bottom of the screen on newer models. Afterwards, tap on the app again to open it. You’ll also want to check for updates to the Facetime app on the App Store, if it isn’t set to automatically update. You may also want to check for iOS updates, which can impact Facetime or the Camera app Facetime uses (via Apple.)
If these aren’t working, you can also trying turning your iPhone off and then back on. If that doesn’t help the issue, you may want to contact Apple Support.