At this age, I think a phone is ok, but it needs to be JUST a phone. No internet, no data plan. Limited texting and, if it's possible these days, no photo messaging. She shouldn't be using her phone all the time, but rather to call you with her whereabouts or to be picked up, to make plans with friends, etc. If she needs to gab away with her friends she can do it on the house phone.
The more she can do on her phone, the more she will disengage with the world around her. My son said someone had his iPad at day camp last week. Camp is only from 9:00 - 12:00. There is NO NEED for an iPad at camp! He's also on the swim team. The other day, there were four kids (older than him, probably about 11ish) hanging out together in the pool. One spent half her time texting rather than talking to the people sitting around her.
So, I think 12 is old enough for a phone, but not old enough for a data plan, etc. It's just not necessary and can lead to A LOT of trouble.
I also think you need to make it clear that any overage charges on minutes or texts will come out of her pocket.
ETA: Sorry, after reading other replies it seems like maybe she wants a phone for her bedroom? If that's the case, absolutely do it. I had one in my room by junior high. Got my first cell phone shortly after college.