"iPhone 15 Plus freezes and delays incoming calls and messages"
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Troubleshooting Delayed Calls:
A. Start with a Restart:
Go here: Restart your iPhone - Apple Support
B. Perform a Force Restart:
Safe what need be, and then perform he Force Restart.
Refer to this link: Force Restart iPhone - Apple Support
C. Update your iPhone:
Install a Cellular Update:
It might be prompting you for a cellular update to install. So, go here for that. Update your carrier settings manually on your iPhone or iPad - Apple Support
and...
Update the iOS:
1. IMPORTANT: If you have an option to update be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update
then...
2. Proceed with the Update:
Go Here: Update your iPhone Using a Computer - User Tip