Last week this problem happened to me.
I followed every step described in several instructional e-mails. Deleted all messages, threads, the contact, reset network settings, powered down and reset my phone, etc. Still had the issue. Gave up until today when I tried to text the number again and....it finally worked. So, my question is, and I've looked everywhere for the answer (unsuccessfully), Why does this happen all of a sudden to a contact? The contact I had trouble with was not a new contact. I had texted the number before, no issues. It appears that it is either due to a) an Apple upgrade; b) an AT&T issue; or c) a combination of both. From everything I've read, all the AT&T employees who have replied only give the canned remedy. So far, no one is willing or able to explain WHAT causes the problem and HOW they plan to avoid it in the future. Just one more frustration you can expect from AT&T !!