I'm a believer in teens having cell phones. My daughters, when they get into their teens, will have 'em zip-tied to their wrists. New ones can dial only approved numbers. You can look up who they're calling, for how long, and when. You can look up their chats. Some, you can even get GPS-enabled.
But beware-- don't get your kiddo a new sexy cell phone, and think that you can cut off his or her Internet access by pulling the laptop from their room. I'm finding more and more underage kids hooking up with older kids/pervert adults, using their web browsers on their phones. These are NOT iPhones, people. This is 1999 technology.
You want to trust your kid? Awesome. Me too. But understand-- that useful tool that is the Internet browser on her useful tool that is her cell phone is a very, very sharp double-edged sword, that can leave quite a mark, if misused.
Just like the gun in my pocket right now-- it's nothing but a tool. But we have to respect tools, and we don't let kids play with them.