NEC DTZ-8R-1 Quick Start Manual page 4

Audio Gain
Offset
TX -6 RX 4
To modify the RX Gain, press the Up or Down Arrow to reach the desired level (+6, +4, +2, -2, -4, or -6)
Audio Gain
Offset
TX -6 RX 4
After you assign the desired TX and RX Gain, press the Select/Enter button