iPhone not receiving withheld numbers

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
• 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/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.
• 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)

Hello Pacer1234,

Thanks for reaching out to Apple Support Communities. We see you have some questions about your iPhone not silencing unknown callers, and we'd like to help. The following link can help explain how to Detect and block spam phone calls.

If you still have issues, please Contact Apple Support.

Regards.