A few days ago I sent a text message to a friend whom I have texted with previously with no problem. She called me to say the texts are coming to her as just a box with nothing in it. It happened to another friend the next day. I called Sprint c/s and they told me to reset my network settings, but that didn't help. I wonder if anyone else has this issue? The two people who are not getting the texts do not have iphones, but I have previously texted them on the phones they have and the messages always went through. All my texts to other iphones are going through fine. HELP!
Another long shot:
Try turning off MMS Messaging. Go to Settings > Messages > MMS Messaging OFF. Then send a message to a non-iPhone and see if it's OK. Make sure Settings > Messages > Send As SMS is ON.