I have a new iphone4s. Friends were asking why I wasn't responding to their messages. It seems that I do not receive regular text messages from other iphone users (the green screen - when we looked at our phones side by side). I do seem to receive imessages (blue screen) when comparing our phones. My phone goes back and forth between sending regular text and imessages - and it seems like all my messages have been sent. Other iphone users messages say they have sent, but I am not receiving them (and it seems to just be the text message with the green screen from other iphone users.) All of my friends have an iphone3, I am the only one with an iphone4s. How can I receive regular text messages from other iphone users?