The question posed should be- how well do you know your teen? If you have raised your child in the right way, then you will be able to predict what he or she will do recreationally with their phone. I know the values of my children (now in their twenties) and I can trust their judgement is good. The most I have ever found out about what they do on texting is my son has accidentally sent me his girlfriends message to me, realized what he has done, and then apologized to me. But the messages were sweet and innocent, mostly" I love yous", or" how ya doing ". And my daughter does not care for guys with " ill intentions". Anyway, my children have an aversion to porn openly because of what they went through personally- and they oppose it( their Dad unfortunately is addicted to porn and they know that is a problem).
So, if you really have to worry about your children it is because you really don't have any idea what they are doing in their social life and you do not trust their judgement. Good parenting means good communicating.
Thanks for letting me answer your question.