iPhone 12 rolls straight to voicemail

Here’s the whole troubleshooting list:

FIRST: Are the 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
• check up on them at Down Detector - outages

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

• Check the MUTE switch on the left side of the phone, above the volume buttons. It should be towards the screen
• Check the volume setting. With the Home screen displayed, press the VOLUME UP button on the left side of the phone. It’s above the VOLUME DOWN button.
• Go to Settings/Do not Disturb. Turn it off if it is on. Also turn off Scheduled. With both off none of the other switches there matter.
• Go to Settings/Phone. There’s a switch for Silence Unknown Callers. If it is on any calls from numbers not in your contacts will go directly to voicemail.
• Also in Settings/Phone: Make sure Call Forwarding is off.
• Go to Settings/Sounds & Haptics. Make sure there is a sound assigned to Ringtone.
• 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 Supportyour 
• If all else fails, talk your carrier into replacing your SIM card (thanks to ASC user DShelbyD)

Go to Settings/Phone and check the setting for “Silence Unknown Callers"

I haven’t seen anything specific to the iPhone 12, except that when it occurs with an iPhone 12 the carrier is almost always Verizon.

Has this been resolved for you yet? I am on Sprint, with iphone 11 pro, and I'm having the same issue. I've tried all the troubleshooting guides, and nothing seems to work.