If you won't let people chat with your daughters, how do you know they won't start chatting with your sons?

So either take away the mobile phones from all your children;

Or

Take the time to supervise your kids when they are using the internet, on the phone etc, and teach them good values.

If you have raised your son or daughter with any common sense at all why would they have given their number to a stranger anyway?

Don't blame the phones, blame the parents.