The Decalc indicator on your Philips Iron is not a button. It is a light indicator that reminds you to descale your iron.
Descaling your iron helps maintain it so that calcification particles do not build up inside the boiler. It is highly recommended that you descale your iron every time the Decalc light blinks. When the iron is cold (usually 2 hours after ironing), open the EasyDecalc knob and let the water flow out into a cup or into the sink.
Please note that if the light indicator is on, it will go off once you descale the iron as recommended. Please find here all descaling information for your specific Philips iron