Interfering with cellular signals is not foolproof. Mobile jammers also have human factors. Blocked signal will affect legal calls as well as illegal calls. Mobile phone reception is completely blocked, and interference will block all phones and SIM cards within range of the jammer.
Guards and other personnel can "accidentally" cut off power or signal jammer. Even with a short interruption, the phone can be connected to the outside world.
The cell phone used for its computer function is working fine and there is no signal. Smart phones can be used well as small computers and data storage devices. Connecting to Wi-Fi can enable ubiquitous voice calls, data transmission and social media.
Prisoners use devices to record messages and videos, then mobile phones or SIM cards. Colleagues pass data and messages to outside the organization or prisoners in other parts of the organization.
