Failed security questions

I upgraded my contract for a new phone. They didn't say they Dont have stock. I past my security questions when upgrading. After finding out a few days later that they Dont have stock I wanted to cancel my contract. Now I'm getting the runaround because I keep failing the security checks. I have gone to a branch authenticated myself with my ID and gotten all my answers. Phoned today to cancel and I'm still failing. How? Can anyone with authority contact me and get past this stupid questions use some OTP or some other method of authentication to cancel this service. Otherwise I'll go to my bank and stop the debit orders