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There is no listed way to change the volume of what this model echoes from the line as it is making initial handshaking. There is a volume setting for the beep the machine makes to signify a change in settings or a button being pushed, but there is no volume setting for the echoing of the tones on the line.
At best, ***** ***** (if you were a technician) disassemble the device, locate the speaker through which such tones are produced, and place muffling material over it.