Just that really. Teenagers are on them constantly these days aren't they? At least my 17yr old DS is. It's depressing me to see him staring at a mobile phone what seems to me like ALL.THE.TIME. I think back to the days when he used to draw pictures and read...........I could go on. Basically, I am that teens, not just mine, are so fixated. Someone will be along to tell me I am old fashioned???
Mine all like to have their phone on them constantly, but it doesn't stop them doing loads of other things. Technology means they keep in touch with FAR more people than I was ever able to at their age, so it can be quite a positive social thing. They do so much more with their phones than older people tend to do, also (sorry, I know that's a bit of a stereotypical generalisation - but then, I guess your OP is too! )
I guess what I'm saying is that they can have their phones constantly to hand, and still read, play football, go out, camp, party, or whatever else it is you'd rather they were doing.