Got written warning for my phone usage!

So ya I'll not lie my phone usage used to be muchh , but when I got verbal warning it has really decreased. Like only for urgent situation I would use. After somedays today I got written warning saying this is last time and chance for me to improve or they will take disciplinary action..I know it's a thing to worry but can you say how big of worry it is? And what actions can they take? I would say I wasn't even using that much phone this days still I got warning. And other employees would use that much . And the email was also sended tagging manager