If i block someone and delete their number how do I unblock the number later

To block a contact from Phone, go to Settings > Phone > Blocked Contacts scroll to the bottom of the screen, then tap Add New. Then tap the contact that you want to block. To add a contact from Messages or FaceTime, go to Settings > Messages or Settings > FaceTime, tap Blocked Contacts. Then scroll down, tap Add New, then select the contact that you want to block. To add a contact from Mail, go to Settings > Mail, then tap Blocked. Scroll down, tap Add New, then enter the contact you want to block.

To unblock a phone number, contact, or email address, swipe left over the entry, then tap Unblock.

You can delete the contact however the number will still remain in the blocklist without the name. You can go back to the block list and unblock it when you want to.

Axel F.