I do not receive certain calls. Silence Unknown Callers if OFF. Certain contacts also don't come through. I've checked everything such as Focus, Airplane Mode and Do Not Disturb. All are off. What could be wrong. My Carrier is AT&T

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 - 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 or later: 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)

If they are not in Recents the calls are not reaching your phone. They still go to voicemail because the voicemail server is in AT&T’s network; the voicemail app on your phone syncs to the carrier’s voicemail server to get your voicemails to you. So this is either a carrier problem or a network connection problem on your phone. Make sure you have the latest iOS version on your phone (16.1.1, as of today) and you have the latest carrier settings. To do that see→Update your carrier settings on your iPhone or iPad - Apple Support

To make sure it isn’t your phone go to Settings/General/Transfer or Reset Phone/Reset/Reset Network Settings. This will erase your Wi-Fi passwords, but will not result in lost data.