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Which numbers get updated?

Any US number missing +1. (708) 555-0142 becomes +17085550142.

Numbers already starting with + are never touched.

Will it affect my international contacts?

No. Anything already starting with + is skipped entirely.

Can I undo the changes?

There's no built-in undo. To back up first: on a Mac open Contacts.app, press ⌘A, then File → Export → Export vCard… Drag it back in to restore.

Why does it need Contacts permission?

To read your numbers and write the updated ones back. Your data never leaves your device.

It shows 0 contacts to update.

All your US numbers already have +1. Nothing to do!