In case anyone is still having this issue, which I had for the last 6-8 months, here's what worked for me. There was just one person that I couldn't text and I'd always been able to until one day I couldn't. I tried everything that's been mentioned on all the forums and nothing worked. Finally I tried putting a 1 (country code for USA) before the phone number (edit contact) and bingo, magic, it worked! It's kind of unbelievable to me that this simple solution is not easily found or I missed it. Hope this helps someone as frustrated with this issue as I was.