Cannot reply to messages

When my wife sends me a text, I have the option to reply by pushing the reply button. When I do the reply screen comes up, I reply and push send. The reply comes back as an error saying it has to be delivered to a 10 digit number not seven. Both our contact lists have ten digit listings in them. If I back out of the message and pull up my contact list and select her, I can send her a text. She can answer my texts but I can't answer hers using the reply button. We only have the one contact list. No other lists,favorites, etc.