Synced or shared iPhone 8 plus

I have an iPhone 8 Plus I am buying from a friend. This phone and it’s service belongs to her. The provider like most give her access to activity now I’m not for sure how much or what she can see. Long story short my settings are constantly changing. After I do a reset I will get a phone call from her. I suspect she has some kind of access to the phone. How could I see if the phone is somehow synced or being shared with her devices.