47 C.F.R. § 64.6100
Minimum and maximum Prepaid Calling and Debit Calling account balances
(a) No Provider shall institute a minimum balance requirement for a Consumer to use Debit or Prepaid Calling for Incarcerated People's Communications Services.
(b) No Provider shall prohibit a Consumer from depositing at least $50 per transaction to fund a Debit or Prepaid Calling account that can be used for Incarcerated People's Communications Services.