My daughter has the iPhone 4 16 GB, iOS 6.1.3 and is unable to text one of her friends. I have read many of the posted tips and tried all of them. I reset the network settings, I turned iMessages off and back on, I did a reset for the phone (not factory as that will be the absolute last resort since Verizon is our ONLY source of data), deleted the message thread, deleted the contact, made a new contact making sure the number included both the area code as well as the +1, typed the number directly into the "to" field and nothing has fixed the issue. I dont know what type of phone or the provider for the friend, but that should not be an issue since its the iPhone that wont send. It shows sending until about 90% and then we get the "Not Delivered" in red with the red !. Also, here lately she is having issues with messages taking days to reach the receiver, even when it says sent immediately after sending it. Im thinking the only other possible help will be Verizon customer support, but I thought I would ask here first to see if there is anything else I might try first. Thanks!