Text replies bounce as invalid number

[Robert Stockdale IV]

My wife first smart phone, a LG Escape 2 has an issue with replying to text messages. She can initiate a text from her phone and receive a reply. However, if the person sending to her has a 7 digit number, even if that person has a 10 digit in her contacts, and she replies it bounces back as an invalid number. Her contacts all are setup with 10 digit phone numbers. I have the Galaxy S6 and do not have this problem. We are on the At&T network. The only thing she is able to do is to delete the message and respond with a new text message. This is not how a "Smart Phone" should work. Any solution to this issue will be greatly appreciated.