Hi!
I would suggest that you consider it wisely. As most have said, it is better to consider the activities that she will be getting herself into that would make her badly need the cellphone. Also, if she has been doing nice, then it can be as a reward for her. If she is responsible enough, I don't think that you can deprive her from that reward. You also have to trust your child.
Also, you can always set some rules to ensure that the phone will be used properly and not just for lame excuses.
More power!