(a) RTT-RTT Interoperability. Covered services and authorized end user devices shall be interoperable with other services and devices that support RTT in accordance with this part. A service or authorized end user device shall be deemed to comply with this paragraph (a) if:

1. (1) It is an RFC 4103-conforming end user device;

2. (2) RTT communications between such service or end user device and an RFC 4103-conforming service or end user device are reliably and accurately transcoded—

   1. (i) to and from RFC 4103, or
   2. (ii) to and from an internetworking protocol mutually agreed-upon with the owner of the network serving the RFC 4103-conforming service or device.

(b) RTT-TTY Interoperability. Covered services and authorized end user devices shall be interoperable with TTYs connected to other networks. Covered services and authorized end user devices shall be deemed to comply with this paragraph (b) if communications to and from such TTYs:

1. (1) Pass through an RFC 4103-TTY gateway, or
2. (2) are reliably and accurately transcoded to and from an internetworking protocol mutually agreed-upon with the owner of the network serving the TTY.

(c) Features and Capabilities. Covered services and authorized end user devices shall enable the user to:

1. (1) Initiate and receive RTT calls to and from the same telephone numbers for which voice calls can be initiated and received;
2. (2) transmit and receive RTT communications to and from any 911 public safety answering point (PSAP) in the United States; and
3. (3) send and receive text and voice simultaneously in both directions on the same call using a single device.