[IgorPartola]

Why don't they put some type of device that suppresses the GSM frequencies near each cell? Not that I am advocating this, just wondering why they haven't done that.

[cdjk]

In the US that would be illegal, and get you in serious trouble with the FCC.

What is possible, however, is for the prison to run its own cell tower with whitelisted IMEIs - i.e. only the guards' phones.