I would strongly suggest that before you start freaking out about your teens' usage of phones and screen-time that you assess the reality of your teens usage and your own! It is easy to get caught in the hype that there is an issue when in fact your teen may have quite a healthy attitude about phone usage. They are 'digital natives' to smartphones, meaning they have grown up with them, unlike us who are digital migrants. Often it is those who are introduced to something new that can freak out about the unknown. Remember when mobiles first came out and there were those telling us we would all get brain cancer?
So first, before you implement a screen regime of the strictest policies, maybe asses what is actually happening.