iPhone 5 (AT&T, ios8.3) Suddenly Can't Text One Number I've been texting my wife's number from this phone for months. Suddenly today I am getting an error message telling me I need to use a ten digit number. The number the error thinks I'm trying is my wife's number but with "11" added to the beginning.

So far I can still text other people without the error.

A similar error error has been reported before. One of the suggested fixes is deleting and recreating the contact and deleting all the texts, but I'd really like to avoid that if I can. I don't want to lose all of those texts.

I tried removing the "1" from the My Phone Number setting, turning MMS off and on, turning off dial assist, turning the phone on and off, manually deleting just her number and entering manually in a text message, and none of that has worked.
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.