Originally Posted by QuasiMondo

All cell phone towers operate on three uni-directional sectors that that are roughly separated at 120*. These prisons could've just gone to the cell phone companies and ask them to swich off or reduce the footprint of the sectors of any site that points at the prison. It's not 100% effective, as a cell could possibly reach the remaining sectors pointing away from a prison (but have poor call quality), but it's much less disruptive than jamming.

Certain technologies, such as CDMA are also more resistant to jamming because they operate simultaneously on multiple frequences rather than just one frequency. The amount of power needed to jam a CDMA signal would be disruptive a more than just cell phones.