Incoming Calls Going Straight to Voicemail

See this comprehensive troubleshooting guide by fellow user Lawrence Finch:

FIRST: 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.
• check up on them at Down Detector - Crowdsourced outage checker

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/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.
• iOS 14 & older: 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.
• iOS 15: Go to Settings/Focus, and verify that all focus settings are disabled.
• iOS 16: Open the Control Center and turn off 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)

You may have a focus mode on or you may want to contact your carrier.