The designer of the comms switching unit could configure it either way round, but I would say that it would be undesirable to combine the whole telephone circuit with the RT voice circuit.
There will be a 2 wire to 4 wire unit in the comms box to separate the incoming 2 wire telephone circuit to feed the received telephone audio to the local headset ear speakers and feed the local headset boom microphone down the telephone line.
The distant telephone user would need to hear if the local ATC had to break-off and transmit on the RT, so it would be useful for the distant user to hear the local ATC boom mic all the time - even when the RT PTT was pressed - so the distant telephone user would know to shut up for a moment. Can't think of a reason to put the received telephone audio onto the local RT voice circuit.
(Hope that makes sense).
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