First, take some time to praise your daughter for recognizing the rules and wanting to follow them. She deserves to be told that she is in the right for being concerned about the rules. I definitley would start with that and give her plenty of praise for caring.
I would not get involved with calling parents or the school. This is the part that will be hard for your good, rule-following daughter to understand, and you want to approach it carefully -- or you risk sending her the unspoken message that "Well, rules don't really matter." You do not want her thinking that, right? So, you asked: Did the teacher see it? No. Does her friend do this kind of thing regularly? Sounds like she might, if your daughter says she likes to rebel. I'd tell your child, "I wasn't there. You were. The teacher, unfortunately, didn't catch X doing this, and it would be better for X if she'd been caught -- she would learn not to do it again. Because she will do it again, and she will get caught and have some strict consequences. Instead of having parents handle this, you are old enough to handle it. You say X is a rebel. You also say she's your friend. Are you really comfortable being friends with her if she is breaking rules and that rule-breaking makes YOU feel uncomfortable and upset? How would you talk to her if you could talk to her one-on-one about this?"
In other words -- your daughter is old enough to work it out or at least try to. She won't be able to change this other girl, but you may get your child to see that they're now at an age when certain friends, even old, good friends, are making poor choices that could get worse. And that the kids who want to be in school and pay attention may have to distance themselves from the kids who....don't. Your child will resist the idea of dropping or distancing herself from this friend, but she at least needs to think about the consequences to herself if she is hanging with someone who eventually does more than have her cell phone in school.
When your girl says "But why won't you call the parent?" it's time to do what we all do when they're much smaller: "Work it out. This is really bugging you, and she's your friend, and I did not see anything. You need to talk to her."
But be very clear that your child is right to be concerned and right to come to you -- because you always want her to come to you with issues and problems, and not feel that "Mom won't care." You need to let her know you do care, but you care enough to have her work it through.
If she had seen the child hurting someone else, or bullying, that would be different; you would indeed contact the school then, right? I just wanted to clarify that that is different from this cell phone rule. Someone is getting immediately hurt and that has to be reported immediately.