National emergency alert system outed prisoners hiding phones
dailymail.co.uk
dailymail.co.uk
The authorities in most countries prefer to monitor, rather than ban or block. Presumably the intelligence gain is worth it, or the risk of riot is not worth it.
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Why? Don't you know that bitter custody disputes qualify as national emergencies on par with the outbreak of a natural disaster or nuclear war? Surely the fact that 99.999% of the time it's only used for the former increases the chance the alerts will be taken seriously.