iPhone not sending regular texts

My 4s has been working perfectly up until today. For some reason, it will only send messages to other iPhones and when I try to sent a regular text message, it ends up saying "Not Delivered". I have done everything possible: making sure everything was on, restarting my phone, calling +228 to adjust my roaming area, etct, but nothing is working. It's also not just my own: friends of mine who have iPhones in general are complaining about the same issue. Buying a new phone is out of the question right now, so please help.