Congratulation on your new family! I have to add my two cents here. I have been a junior high school teacher for almost 20 years and have a first hand view of teen behavior and phones (especially while out of view of their parentals) While I understand the parents who have responded saying, "if that's your biggest worry, be happy", I have to agree with those who have responded with, "set boundaries".
The cell phone may seem innocuous, but it can cause a huge amount of trouble. For sure keep it while she is doing homework, and in the evenings. Explain she is not to use it during class. It should stay turned off (not on vibrate) and in her backpack during school hours, and teach her how to text kindly and speak as she would face to face with peers.
The thing about texting is that everything she writes can be spread like wildfire across the town with a click of a button. One spiteful moment by a friend and everyone in school could know she has a crush on a particular person.... It can be quite damaging to the teen who is not careful about what she shares. Same with personal information and any moments she has with friends i.e." she was such a b*** today" to a confidant, could turn into a huge "diss" fight with a whole crowd. Then there are all the texts sharing school info, which can turn into sharing answers and end up in school discipline, the texts explaining "mom and dad are away at..." that turn into a whole herd of rowdy students ending up at your house and the texts that express," I'm kinda loney or sad.." which can turn into offers to hang out (good) or share drugs (omg).
Without face to face communication, a ton of information that helps our teens understand what's being said and shared is lost. We need to use texting minimally. Would you let her talk on the phone all day? It can be too addictive. Cut the cord to the friends line, and allow teens be PRESENT in the space they are in. She needs time to bond with you as a Mom too. Having said that, I completely support the moderate use for staying connected with where everyone is, but be aware and do set the limits. It can make a huge difference in the success and health of a teenager socially and with the family.
Good luck!
Erin