How do I unblock a number on iPhone that I did not intentionally block?

Settings->Apps->Phone->Blocked Contacts->tap Edit and select the number you wish to unblock and tap the - (minus) sign.

For future questions refer to the iPhone User Guide.

iPhone User Guide - Apple Support

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

Go to Settings > Apps > Phone, then tap Blocked Contacts. 

Find the number you want to unblock and swipe from right to left and tap Unblock.