how to dispute bank fee/poor credit?

Q. I never have missed or have been late on 1 payment ever. Last
month I used MBNA’s phone pay service to make my min payment. I got a
letter today that says my account is overdue because my phone payment
bounced.
I called them today and they told me that the payment bounced not due
to NSF, but because the checking account I entered on the phone does
not exist!?!?! We verified the checking and it is perfectly valid. I
double checked with my checking bank and it is the correct account.
MBNA charged me…ready for this rip off… $29 late fee, $50 returned
check fee, $50 account overdue fee. Plus they reported on my credit
report a 30 day late item which knocked 50 points off my FICO score.
They flat out won’t remove the charges or late item claiming the
checking account I entered does not exist and it was my fault.
What is my course of action at this point???
A. -This makes me wonder whether we’re being told the whole story.
Usually banks will give some leeway on fees the first time a customer
makes a mistake. (Note on strategy: even if you think you didn’t make
a mistake, it’s better to say you did. Ban