2morecents, a parent should be fully aware of who their child is friends with and what the positive and negatives of each friend are. Again, monitor your children’s texts and phone calls (the numbers are easily traceable on the bill). Know your childrens friends. Know who they are hanging out with, where they are going, what they are doing.
Unfortunately, its fear mongering like what you are talking about that has laced Judaism today. Owning a phone is not going to lead to licentiousness, especially if you are careful. If you are irresponsible parent and let your children run wild, don’t verify where they have been or what they are doing, then maybe. But a phone is a tool, nothing more.
As to married adults – I have had my phone since before I got married. In 6 years, I have not used my phone for impropriety.