Panasonic KX-TD500 System Reference Manual page 1247

Item
Number
Voice Level(Transmit)
3
Voice Level (Receive)
4
Balance Network
5
Valid when "Voice Path Type" is set to "2:4-wire".
Conditions
This command is valid only when E&M card(KX-T96184)is installed to the system.
Assigning Items
11-C-121
Input Value
X(1 to 4)
1: -6dB
2: -3dB
3: 0
4: +3dB
(Default = 2)
X(1 to 4)
Should be fixed to "1".
(Default = 1)