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I assume you are looking at the markings on a power supply? and if so then they will relate to the maximum current draw when the power supply is actually delivering its full rated load current. That is something that will never happen when powering the ONT.
Those figures do not mean the equipment it supplies will consume anything like that power. If it were drawing 0.5 A @ 240V then that would be 120 watts which is a lot of heat, certainly way more than the plastic enclosure of the ONT could handle plus it would all be to hot to touch.
I would guess a realistic power consumption figure might be 10 to 15 watts for the ONT.