blue versus grey phone icon

How can the phone icon be blue almost constantly in messenger when the last date active was 3 days ago. It went off periodically (turned off overnight) and was back on the next day. Status still says active 3 days ago. If one logged out and back in you would think this would change the active time. Does the icon simply turn blue if one is using their phone or if they've made a fb phone call? I would think a call would change active status as well.