Hello Shane96,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with your iPhone 12 not ringing consistently or sounding alarms when they are scheduled. Which exact version of iOS are you updated to under Settings > General > About? Does restarting resolve the issue? Are the callers in your Contacts app? If not, check to see if Silence Unknown Callers is turned on as described here: Detect and block spam phone calls
Also, check to see if the steps here resolve the issue: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
We hope this helps.
Cheers!