If you plan to record a video that is important to you, set your phone to 'Do Not Disturb' to prevent an incoming call from interrupting your video recording.
Here's how to set your iPhone to 'Do Not Disturb':
Go to Settings > Do Not Disturb and set Manual to 'on'. You can also adjust your 'Allow Calls From' setting to 'none'.
Or Swipe up on your Home Screen and select the moon icon, which turns 'Do Not Disturb' on.
Make sure you turn 'Do Not Disturb' off when you're done recording the video or you'll start to miss calls.