A 12 year old is old enough to date, but now own a cell phone? Really she's mature enough to know how to handle the emotional and possibly physical aspect of a relationship, but not the responsibility of a phone?
Most middle school kids have boyfriends/girlfriends? Not where I live, and just because most do doesn't mean yours/mine should. That's like saying "Most middle schoolers are smoking pot, get over it".
You won't be able to stop this, so why bother. When in Rome, join them! That's kinda ludicrous, who's the parent?
If you don't want her dating, then make that very clear to her. Explain your expectations, and explain the consequences should she break those expectations. She should have known where you stood from early on, but it's not to late.
You also, can't stop her from seeing this boy in school, but it's not like they have hours of free time together. What a break and lunch? Fine, but anytime outside of school is a complete no-no.
Mine didn't date till 16, only one tried anything and she didn't like having to be chaperoned everywhere, so she learned her lesson and waited. Oddly, she has thanked me for that. She could see, later down the road, that she wasn't ready.
Figure out what you can and can not live with. Then stick with that.