Once a number is blocked on a cell phone, it is not permanent. Smart phones come with a feature to block calls, but they can be unblocked.
To unblock a number, users have to go through the settings applications in their phone. For example, in the iPhone 5, users go into the "Settings" application on their phone. Users then scroll down to "Phone." In the "Phone" section, under "Calls," there is an option called "Blocked." Once in the "Blocked" category, a list of numbers that were blocked appears. Users can then edit the status of these numbers and unblock them.