I am also having this problem. I have read numerous threads on this issue and I have tried all suggested solutions except, I have not deleted all my text messages and I have not reset/restored my phone. I have an iPhone 5s and recently ported my service to AT&T. One thing I have noticed, which I don't know if it is applicable or not, when I search for the number I am trying to text to using spotlight search, I get search results. Even though I do not have any text messages to/from this number, nor do I have a contact saved with this number. Further I do not have any group messages with this number.
I suspect at one time I had this contact in as a 7 digit number, may have existed that way when I was with VZ. The NPA/NXX of this contact is assigned to VZ. All recent attempts to text to this contact has been using a 10 digit number, however the reply always references a 7 digit number.
As stated by others I can call this number, the contact can call me and the contact can text me. I cannot reply to a text message or text via a new message to this one contact. I suspect, although I'm not a network engineer, that there are two issues. 1. AT&T can't route the number as a 7 digit number, which I understand. 2. Somewhere in the iPhone the 7 digit number is still being referenced even though all traces of the number have been deleted (other than search results in spotlight).
I'd sure like to be able to resolve this issue without erasing all my message threads, which I'm not confident will fix the issue, or reseting my iPhone.
Thanks in advance.