I would suspect that it has something to do with your phone's settings. Sometimes in options is the ability to block all txt messages, or even set them to only receive texts from contacts or those you have manually added. I would search through the phones settings and see if that helps. If not I would also look for a different area these text messages have been sent to, also while testing ask one of your friends if they get the "message not sent" error.
Hope that helps,
Nate