Welcome to the community!
If you blocked the phone number from your device, you should be able to unblock it from your device as well. What phone are you using? If you go into your Call settings, there should be a section for Reject calls or Block calls. Within that section there should also be a list of rejected or blocked callers. You should be able to then find the number in the list and uncheck or unselect the phone number for blocking.
Hope this helps