I had this problem at first after I was promoted to SSV—I’m a very non-confrontational individual, so it took a lot out of me to become more authoritative with my partners. I’ve had the phone issue come up a few times, and I usually say “don’t do that on the floor. If you gotta take a message or call, go to the back room if there are no customers out here/you can get your position covered.” Tone of voice matters. Learn how to say something politely/light-heartedly at first, and then become more forthright if you must. In the end, you’re the supervisor, they’re the partner, and if they are consistently breaking store policy (on their phones on the floor) and ignoring you, that’s two write-ups—insubordination and the company policy in question.

At the end of the day this is a job first, and friendship is a secondary bonus. At all stores I've worked at we never fully enforce the no cell phones rule, but we also never have them out in front of customers. It's all about finding the best implementation of the rules to keep staff happy while still presenting a good face for the customers. Don't threaten your firned with a write up immediately, just casually remind her not to have it out if customers are present, and if she keeps doing it remind her more sternly once or twice more before flexing some authority and threatening to report or write her up. If this barista gets pissy about you doing your job and reminding her not to use her phone in front of customers then she's not a great friend and not a great employee. She's gonna have to learn professionalism at some point, even if she (and many others) don't consider being a barista all that professional. Your SM has specifically told you to enforce the rules, which means they are aware of the issue and don't want to handle it themselves (cowardly, but whatever) so this is a test to see how you handle yourself. Good luck!

I have this issue quite a bit with baristas cause I'm the "nice one" and don't care for much lol but you do have to step your foot down and be like "that's a no no, sorry but I don't want you to get in trouble if a customer sees you on your phone and calls corporate over it" then try to give them a small task to keep them occupied. You might get a small amount of hate but just let the barista know you want the best for them and the team!