Surely as an instruction from an ATC unit, it should be read back to the station to confirm the correct squawk. I understand your intentions/validity of not replying but the Iran Air incident is a case in point where the correct squawk was vitally important.
As to the correct response to the call, I think it's splitting hairs. There should be a correct phraseology for this but I believe "Ident(ing) XXXX", "Squawk XXXX" even "XXXX Coming Down" seem to be internationally recognised by the ATC stations I have spoken to.