I had cell phone contracts with my girls. Only a couple of times did we have any issues - mostly with not putting the phone away at the designated time. We also required them to charge the phones overnight in the kitchen as opposed to their rooms.
I'm sure they got away with things, but they turned out fine. I would not resort to reading their text messages without them knowing. If I had a concern, I would have them bring me the phone to look at in their presence. I don't remember ever having to do that. At one point, the one of the phone passwords was changed, but that was a result of Apple moving from a 4 digit password to 6 digit password.
The girls' phones were linked through my wife's apple account. They could not download any apps without her permission. We were able to discuss the apps with the girls prior to allowing them to download.
Our relationship with the girls has always been trust until they give you a reason not to. YMMV