I'm not getting ringing for FaceTime and not getting alert sounds when IMessages come in. I believe I've got all my setting right, and I can see the calls coming in if the iPad is in use. Do not disturb is not on, volume is not muted, the apps all have sound settings, etc.
I have been having the same problem. I won't know if it works 'till someone calls - but I have just found in Settings>Do Not Disturb>Allow Calls From, change to EVERYONE.