This happens more often than you would think, and it is usually a simple fix.
Most inmate phone systems have a feature where pressing a certain key during an incoming call, often a number on the keypad, triggers an automatic block on that number. It is easy to do accidentally, especially if you are fumbling with the phone or hit a button while trying to answer. The good news is it is not permanent.
Call the facility directly and ask to speak with the counselor on duty. Explain that your number appears to have been blocked and that you did not intentionally block it. This is a known issue they deal with regularly, and a counselor or phone administrator can manually override the block on their end. It typically does not take long once you reach the right person.
If the counselor cannot help directly, ask who handles the inmate phone system at the facility. Some institutions use a third-party phone provider like Securus or ICSolutions, and those companies have customer service lines that can also remove blocks on your number with verification.
Have your phone number ready and be prepared to confirm your identity when you call. The override is straightforward once you get the right person on the line.