iPhone 12 Pro Max Silenced When Locked

Some of the reasons are listed below which may affect the incoming calls

1. Bed Time schedule
2. Call Audio Routing method
3. Ringer Volume level
4. Call Forwarding to another number
5. Silence Unknown Callers [Enable] Calls status: Only numbers in Contacts, Mail, and Messages ring your phone. All others are diverted to VM
6. Have you done any of these; decline a call and send it directly to voicemail, by doing one of the following:
   1. If iPhone is unlocked, tap Decline.
   2. If iPhone is locked, press the side button or Sleep/Wake button (depending on your model) twice quickly.
   3. On your EarPods, press and hold the center button until you hear two low beeps.
   4. To be reminded to return the call, tap Remind Me, then choose when you want to be reminded.
   5. To respond with a text message instead of answering, tap Message, then choose a reply or tap Custom.
   6. To create your own default replies, go to Settings > Phone > Respond with Text, then tap any default message and replace it with your own text.
   7. Block unwanted callers: In Favorites, Recents, or Voicemail, tap  to see a caller’s contact card, then tap Block this Caller. Or go to Settings > Phone > Blocked Contacts. You don’t receive voice calls, FaceTime calls, or text messages from blocked callers. All are diverted to Voicemail

DND is disabled then the two options under the Silence Section are irrelevant.

Look for other settings which are silencing your incoming calls,