I once attended an internal presentation while working for the UK's Ministry of Justice.
A large number of contraband mobile phones had been confiscated, and a team performed some data analysis to see what they'd been used for.
The overwhelming conclusion was that the phones had been primarily used to keen in touch with family.
There's also a whole bunch of research that showed that maintaining ties with the outside world while incarcerated led to reduced rates of reoffending (and the inverse was also true - isolation led to increased rates).
Allowing free phone calls in and out of prisons makes a lot of sense both socially and economically.
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