iphone 12 rings on some but not all calls

Are the ones that go to voicemail not in your contacts, perhaps? There’s a setting that sends calls from numbers not in your contacts to voicemail. Go to Settings/Phone and check the setting for Silence Unknown Callers.

If that isn’t the problem, here’s what else to check:

Most basic first: Have you checked the mute switch on the left side of the phone above the volume buttons, and adjusted the ring volume with the volume buttons while viewing the Home screen? (You must be on the Home screen to adjust the ringer volume). 

If it rings but the ring isn’t loud enough: 

• First check the speakers. Go to Settings/Sounds & Haptics/Ringtone/Classic and tap Old Phone as a test. If it is loud enough then change it to whatever you want.
• Go to Settings/Face ID & Passcode, enter your passcode, and look at the switch for Attention Aware Features. When this is on sounds will be lowered when the phone sees that you are looking at the screen.

If there is no ring at all:

Are there missed calls in Phone/Recents on the phone? If they are not the problem is in the network, because the calls aren’t getting to your phone. Contact your carrier: 

• check with your carrier that they don’t have a local outage, and that you didn’t have any settings that would have immediately invoked voicemail, such as call forwarding.
• Note that they will still go to voicemail, because voicemail is prompted and recorded in the carrier’s network, not on your phone.
• check up on them at Down Detector - Crowdsourced outage checker

If they are in Recents here’s what to check next:

• Settings/Phone: Make sure Call Forwarding is off.
• Make sure Do Not Disturb is off:
  • Open the Control Center and turn off the button next to the word Focus.
• Go to Settings/Sounds & Haptics. Make sure there is a sound assigned to Ringtone.
• Go to Settings/Accessibility/Touch/Call Audio Routing and make sure Auto-Answer Calls is off.
• Go to Settings/Screen Time/Communications Limits. Tap each of the 2 entries and make sure both are “Everyone"
• Restart your phone-->Restart your iPhone, iPad, or iPod touch - Apple Support
• If all else fails, talk your carrier into replacing your SIM card (thanks to ASC user DShelbyD)