We'd know, except the two biggest companies GTL and Securus, that together make up about 70% of the market, are both owned by private equity and have no requirement to disclose.

It is known that a high percentage of fees are paid back to the prison the calls originate from [0], and that offering to return a higher percentage is one way these companies win exclusive contracts with prisons.

So color me suspicious that a profit source for prisons is being run at the most efficient and lowest possible cost to inmates...

[0] https://en.m.wikipedia.org/wiki/Inmate_telephone_system#Serv...