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/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)
