I have had my iPhone since August, and it won't text one particular person. I can text everyone else and they can txt me back and even this person can text me I just can't text back. We thought it was his phone but he just got a new phone and a new number and were on the same provider and still won't text him. Which is weird because its even a new number. What's going on? We also took it in and the lady just took out the SMS card and we could text for a day that's it. Then it went back to not being abled to text him again.
I had this problem too. After resetting the network settings, changing the number as set to iPhone insted of mobile and deleting it and putting it back in and deleting the previous texts it finally worked. I can't say which one of these fixed it though.