If there is one particular contact number that you are having issues with, such as being unable to reach them via calls and texts when it used to work previously, there is a possibility that your contact has blocked you. Alternatively, you may have accidentally blocked them as well. This is especially true if you are still able to send and receive texts from all other numbers as per normal.
To check that you have not blocked your contact accidentally, follow the steps in: How to block someone from calling or texting me?