I've heard other parents say that they've monitored their kids access via services available from their phone provider; is this true? As for the internet stuff I believe I've already got that covered. I've also heard that text messaging can be monitored. Is it possible to control which apps they install as well?
Give them your old iphone and don't let them change the password. Help them understand that the internet is forever, and what is and is not appropriate. My daughter isn't really texting addicted, phone addicted or game addicted, but if it became a problem I would confiscate that sucker in a heartbeat. A phone is a privilege. And set limits - no phone at meals. No phone when the family is doing an activity together. And no phone right before bed - the bright screen can cause insomnia. And if they loose or break the phone they aren't getting another one. It can help teach responsibility and proper possession control.
If your child has an iPhone you can use the same Apple ID so that all his iMessages go to your phone as well. But problem with that is any android message will not go through, and they could also shut that off on their own. iPhone is good with its restrictions though. You can set it up so they can't use Internet, can't download apps that aren't age appropriate, and a whole lot more as well.