This is a sticky situation! I know you are upset, but it is for selfish reasons. Plain and simple, you need to just let it go. If you don't, you'll end up bitter for no reason. It's just a phone.
The thing you are upset about has nothing to do with the actual phone. You are feeling like your rights to choose her cel phone are being infringed upon. And, you're probably feeling like someone infringed upon your special plans and what you perceive to be your right to be the only person to get a cel phone for your daughter, which translates to a feeling of lack of control over your daughter's life.
In reality, your ex- has as much right to choose and give a cel phone to her as you do (no matter where she lives). The only right you have is to say whether or not it enters your home. So if you are upset enough to insist upon this, you can ask that it be left at his home while she is with you. If that does not work, you can always ask that she check it in with you while she is at your home. I warn you that this probably will upset your daughter (at least at first), and you need to weigh whether or not that is worth it. If you do go this route, you should also expect that she won't use his phone to call you...and maybe won't use yours to call you either. He may have her check it in as well.
I am a stepmom. From the perspective of someone on the "other side", they probably did not know you had a cel phone planned (and if your daughter was asking for it from you...she probably asked for it from ANYONE who would listen, dad included! All kids do this.)
Personally, I would just let her have the cel phone and return mine - or keep it and let her have two. I would tell her that we could have a memorable experience as in family outing or mom-daughter outing instead, spending just as much money. Speaking from experience, the cel phone will be history in her mind when she is 30 years old. If you take her horseback riding or to an amusement park or just out shopping...THAT is going to stick with her! FOREVER.
I did not mean to be harsh with you. You are entitled to be upset. The divorced thing can really throw you for a loop, and it's very hard to have to be the more grown-up in a situation where the ex- may be continually undermining you and not acting mature. But you need to let it go for your daughter's sake! She will thank you for it later. You will also be proud of your actions later, for taking the high road.
Also, keep your head up high and even if you need to pretend, just act like nothing is wrong. Be gracious and ignore the actions of your ex-.
Blessings to you!