iPhone 5 (AT&T, ios8.3) Suddenly Can't Text One Number
I just fixed this after 2 years.  First- delete the contact altogether.  Second - delete every single message the person has ever sent you, including any group texts they are in (if it's not obvious to you, search for their name, and when the messages come up, swipe left on them and hit delete).  Third- re-enter their contact, being sure that there is a 1(xxx)xxx-xxxx format and that the selected type of number is iPhone (even if it Is not). 

Send a test message.  If it doesn't work, do a reset and do it all again.  If you don't delete every single old message, it won't work.  It worked for me!!

I have no idea what the original problem was because I share contacts with my husband and he could text these two contacts.  I just know this worked.  Apple tech even said she was going to write it down to share because she had no idea.