iPhone rings once and then stops ringing. All stupid awareness settings are turned off. Still not ringing and missing calls. WHY? How do I fix?

Some of the reasons are listed below which may direct your calls to VoiceMail

1. DND status
2. Bed Time schedule
3. Call Audio Routing method
4. Ringer Volume level
5. Call Forwarding to another number
6. Silence Unknown Callers [Enable] Calls status : Only numbers in Contacts, Mail, and Messages ring your phone. All others are diverted to VM