If you recently blocked a caller and want to undo it quickly, use the Phone app. How To Unblock A Contact Or Phone Number On iPhone

This is the standard way to view and manage all your blocked contacts in one place. Open the app on your iPhone.

: Swipe left on the number or contact and tap the red Unblock button.

This is the most reliable method as it acts as a central hub for all blocked contacts across Phone, FaceTime, and Messages.