The main complaint and cost for prisoners as I understand it is the high cost of calling family members. Where most people might not even remember pay phones, if you do at their peak it was $0.35 to make a call, versus $6-24 for 15 minutes in prison [2].
When you factor in that most prisoners, that can even find a job in prison, make around $0.10/hr and most of their friends and family members are struggling on the outside bc they are just poor or at least one of the bread winners is now making $0.10/hr, that $15/call is very expensive. And even those that run the prison systems will tell you the number one thing to keeping a prisoner calm is regular communication with their family and friends [1]. It also costs $0.35 to send an email, while the two companies that run the telecoms for the prisons in the US raked in $1.4B [3]
Sorry my rant wasn't aimed at you, just wanted to clarify the part about batteries.
[1] https://www.vera.org/news/prisons-and-jails-keep-making-it-h...
[2] https://ellabakercenter.org/wp-content/uploads/2022/09/Who-P...
[3] https://www.businessinsider.com/high-cost-prison-communicati...