TechPro intercomm stations are connected together using standard XLR microphone cables. The pinout used is

The signal carries audio, which is about 100mV RMS. The calling function is superimposed onto this line; when the call button is pressed the audio line rises to a +12V offset from ground. The rise is slow, over a few hundred milliseconds, which prevents an audible click on the line, however you can sometimes here changes in noise level when the button is pressed.

Each TechPro signal line is terminated in the power unit, a schematic of the termination is: