BYOB phone from straight talk & refuses to unlock!

My daughter was on straight talk for 4 yrs. Actually both of my daughters were. We put our youngest on our att plan last year when her sister moved out. She kept up with it until recently. Her previous phone she had the whole 4 years broke. I bought her another one from straight talk & had it shipped to her a few months ago. She then had some issues. Came home, and couldn’t pay the bill so we added her to our account too. Got the sims card today & found out her iPhone is locked. I understand a contract device being locked but not one paid in full. That should be hers to do as she pleases. I never even knew of this. I would have never signed them up for straight talk. And I will not use them again. Anyone else have any suggestions or had any success?