Early Termination Fee

I had a company account for 5 years with ATT. When I left the company, they moved the account to my name so I could port the number to my new company (only the number was moved - I did NOT take the phone with me, it remained with my previous employer). I have since been hit with an almost $300 early termination fee. I never intended to keep ATT service so I never would have agreed to a contractual period. After SEVERAL call to billing I’ve had no success getting this removed, not only that no one can prove I agreed to a contract. Now I have collections calling and they’re going to hit my credit unless I pay this fee I do not feel I owe. I’ve tried calling, I’ve been told I’d get a call back from a manager which never happened, I’ve sent letters and now they’re asking me to visit a store in person. What can I do to resolve this? If I am in the wrong, and ATT can prove I agreed to pay this fee I will but no one has been able to even answer whether I agreed to it. It’s frustrating knowing that my credit is in jeopardy over a bill that I don’t even owe.