Who do I contact for a large credit?

I was told that a credit would be issued to my account for three months service.  This was back in Feb/Mar and I was told that it would appear on my bill in 1-2 billing cycles.  It has not appeared and I've called in multiple times and been told the same thing everytime, that it would be escalated to a supervisor and I would hear back from someone in 24 hours.  That has never happened and today I was on the phone for over 2 hours and still got no where.  Today I was told that what I've been told all along is not Comcast's practice.  I was also told that the call center I reached only had a credit limit of $300 that the request could be submitted and it would take 1-2 billing cycles.  After repeating myself way to many times and trying to explain that this is what was done way back already, I still didn't get anywhere.  I was told there was no one higher than the person I talked to and that Comcast does not call customers back.  Seriously??  I was told I could call back but I would probably only end up talking to the same person again.  I asked to be transferred to another department of customer serivce and waited on the line for 45 mins and no one ever picked up!!!