NEC DTZ-8R-1 Quick Start Manual page 3

The default setting for Transmit and Receive Gain is 0
Audio Gain
Offset
>TX 0 RX 0
To modify the TX Gain, press the Up or Down Arrow to reach the desired level (+6, +4, +2, -2, -4, or -6)
Note – NTAC has found that setting this value to -6 has resolved issues with echo. Your adjustment
settings may differ with your particular environment.
Audio Gain
Offset
>TX -6 RX 0
To modify the RX Gain, press the Right Arrow button.