How to find out what date a blocked caller called

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Feedback - iPhone - Apple

It is not possible to see the date when you blocked a phone number. However, you can check the list of recent calls in the Phone app to see if you recognize the number. Time will appear for each call, after a day the date will appear.

You can also see the list of contacts you have blocked here:

Settings → Phone → Blocked Contacts

You can find more information about blocked contacts here:

Block phone numbers, contacts, and emails on your iPhone or iPad - Apple Support