I'm not sure that total deregulation of the prison phone market is a good idea, but at least in the United States, the current solution is pretty outrageous [0]. There is a a reasonable censoring role to be played by the prison (same as it is for mail), but like much else about prison system, seems served by rent-seekers [1].

[0] https://theintercept.com/2017/06/16/fcc-prison-phone-call-ra...

[1] https://www.indystar.com/story/news/crime/2017/11/20/indiana...