I would think jamming certain frequencies for a certain distance would require FCC approval.
No need to jam all the cell frequencies.
All that is needed is a set of receivers on the prison grounds that can detect and locate cell transmissions.
Any transmission that comes from the prison itself can be selectively jammed. It will not cause interference even to someone walking by the front of the prison who is using their cell phone.
Not an engineering miracle, just simple application of existing equipment.