why does my Iphone 7 shut down calling or receiving a call or texting

Force restart iPhone - Apple Support ⬅︎⬅︎ try this... ⬇︎⬇︎

1. Press and hold both the volume down button and the Sleep/Wake button.
2. When the Apple logo appears, release both buttons.

And/Or...Update your carrier settings on your iPhone or iPad - Apple Support

Also...Reset Network Settings...

From a Home screen, navigate: Settings > General > Reset.

Tap Reset Network Settings.

If prompted, enter your passcode.

Tap Reset Network Settings.

iPhone 7 Repair Program for "No Service" Issues - Apple Support...this may apply to your phone...