How to unblock numbers

Settings>Phone>Blocked...tap the "Edit" button, top right, then tap the red minus sign next to the number you want to remove from the blocked list, tap done when finished. The number will be un-blocked.

Note that any number that has been blocked can just as quickly be unblocked via the same steps.

From here...

http://www.tuaw.com/how-to-block-phone-numbers-in-ios-7/

What if the number isnt showing in your block list anymore but it still wont let you save the nunber?

Did you read the Article ?