What happens if my daughter buys a phone on my T-mobile account without permission?

[Karen E]

My husband and I have a T-mobile account. My daughter (18 yo) also has a line on the account but she is not authorized to change the account etc. She isn't a primary account holder. My daughter went to a T-mobile store and bought a brand new phone without permission. We now have a 266.00 dollar equipment charge on our account. She states she didn't sign anything but I don't really know. We suspended her account and she has 30 days to take back the phone or they will charge her with "fraud". Well, she went to T-mobile with her phone and opened her own account. There is still an equipment charge on our account! If she didn't sign anything is it T-mobile's fault? What should we do? Thanks