47 CFR § 64.6040
Rates for calls involving a TTY device
February 16, 2021
(a) No Provider shall levy or collect any charge in excess of 25 percent of the applicable per-minute rate for TTY-to-TTY calls when such calls are associated with Inmate Calling Services.
(b) No Provider shall levy or collect any charge or fee for TRS-to-voice or voice-to-TTY calls.
[80 FR 79179, Dec. 18, 2015]