I'm stuck on my moms phone bill, which she owes 600 dollars on. I found out today and I don't know what to do.

My mom (55) is currently in rehab, she called me the other day telling me (22 f) I have to pay half the phone bill, and today she called telling my I would I have to pay the full phone bill. I call AT&T to see what the balance was which was over 600. There has been constant late fees and she has been depending on me to pay the phone bill. I called AT&T and they said my mom has to give consent of me getting off her plan to get my own. Or even if I get my own plan she still has to give consent of canceling my phone. I am on the account but haven't signed any papers saying that if my mom doesn't pay the bill they will come after me. But since I am on the account I'm afraid they will come after me to pay the bill that I was very unaware of. What legally is my responsibility? I'm in Utah