If you aren't connected to the cellular network, you won't be able to make calls. You can check your network coverage at the top of your screen. If you're out of reception, you may be in a dead spot or there may be a network outage, so try moving around a bit.
Why is my iPhone not making calls?
Go to Settings and turn on Airplane Mode, wait five seconds, then turn it off. Check Do Not Disturb. Go to Settings > Focus > Do Not Disturb and make sure it's turned off. Check for any blocked phone numbers.
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