Hi, I've been having problems with the "text message not sent" notification, and I've found a fix that's been working for a while! the phone number you may be texting could have a number such as 1-XXX-XXX-XXXX and I found starting a new thread under that same number, but without the 1 in front, and all of my texts have been working as of yet! the Facebook app added all of the people on my facebook into my phone, and added the 1 in front of the number, and this may be happening to you. So try to create a new thread with the same number but without the 1 on front.