To unblock any phone number that you have manually blocked, go to the “Block” tab on the app. Find a row containing the respective phone number or the range of phone numbers that you are wanting to unblock. Swipe left on that row.
If you swipe left completely on that row, that phone number or the range of phone numbers will get unblocked. If you partially swipe, it will give you an option within that row to unblock the number(s) on that row.
You can also unblock all the numbers at once. To do so, tap on the delete icon on top. Then, select whether you wish to block all individually-added numbers or all range-blocked numbers. As soon as you select an option, the respective number(s) will get unblocked.