You can have your cake eat it too… that specific employee has a brand new cellphone policy starting at your next 1:1 I think. No phone until task completed. Act like a child get treated like a child. If they don’t quit or your problem isn’t instantly resolved, I dunno what, but I’d start there. As others have said transitioning into a new job is stressful, whether or not the work itself is, just due to all the surrounding logistics that complicate life when people are generally seeking employment. Be direct and open and honest with them, your policies sound like a dream to me if I were in those shoes because I need music to multitask. No one under the age of 40 feels complete without their phones in their pocket, whatever that says about our society 😂, they’ll be done first for the following week every day and then you can go back to normal. Employee will learn the lesson and appreciate the opportunity to grow when they look back on it, or they won’t and they’ll go waste somebody else’s time and money.