If you're using the Google dialer:
Open the phone app, tap the 3 dots in the top right corner, tap settings then call blocking. That should be a list of blocked numbers, tap the X to the right of the one you want to remove from the list.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Android Central.
Facebook
Twitter
Instagram