Wrong amount charge, customer service not responsive, charged 6x more than what I usually pay

I was charged with the wrong rate for the past 3 months.  Called a few months back to complaint and informed I will get a credit back which never happened and then I received the latest statement with outrageous amount on it.  I usually paid around $130 monthly for my bill and for this month I was charged $750!  Called 3 days in a row and said the person that can help me was not available.  Called the 866 5998-9374 many times, they all told me that someone with the name of Breanna is the only person that can resolve my matter. Left a message with a supervisor and did not get a response back too.  Who should I escalate this issue to?