Inmates control their own approved phone list. They can add and remove numbers themselves through the GTL phone system at the facility, typically using the kiosk or the phone unit interface, without needing staff involvement to make those changes.
If your person has forgotten what numbers are currently on their list, the simplest way to get that information is to ask their counselor. The counselor can print out a report of the current approved numbers on file. That gives them a clear record of what is there so they can make any changes they want from an informed position.
This is one of those things that is easier to manage from the inside than most people on the outside realize. The list is the inmate's to maintain, and keeping it current is worth doing, particularly if circumstances change and certain numbers no longer need to be on it.