What's your issue about this? Is it that you think there is some kind of legal right to property here which you think you need to enforce? If so, is this because you are a big legal enforcer in all areas of life, or is it because you are someone who generally resists school rules and any kind of discipline of your child and so your annoyance over the phone thing is another way to show disapproval of the schools approach? Have there been other instances of you making a case against the schools approach towards your child?
I just ask these questions to try to get a sense of what the issue is here and your approach to school. Are you generally a parent who is supportive of the school and its policies, both generally and when applied to your child, or are you someone who is always looking to complain and thinks the school is out to get your child or that any rules shouldn't ply to your child and you will try to get them dis-applied when applied?
So, if you are generally a very supportive parent who backs up the school, and think that the approach shown over this phone has been reported accurately by your child and doesn't reflect the school policy properly, then it might be worth emailing with a query about what has happened and the school policy on mobile removal. A query always works better than a complaint or demand.
If however, you are a parent who is regularly in conflict with the school and annoyed by its policies and challenging them, I'd suggest just thinking a bit more carefully about how you approach this.
Schools do make mistakes in terms of applying their own policies and in terms of discipline. Quite often, with a minor example, the best approach is to teach children that sometimes these things happen and that one of life's lessons is just getting on with it and accepting it, rather than always wanting someone to wade in. Of course, if there is a more serious example or it is regular, then parents should get involved.....again, a polite query is usually more productive than a big rant.