Billing Issue

We had service with AT&T that stopped working in June of 2018 but AT&T continued to charge us.  Technicians were out multiple times trying to fix and we were finally told that AT&T could not support our facility.  I was told when we cancelled our service in October that I would receive a check for $719.34 to credit us for all of the months we paid with no service.  We only received a partial credit for some service and some equipment.

I have tried contacting AT&T multiple times, each time being switched around, being hung up on and holding for 45 minutes or more.  I detailed documentation of the issues and the credit I was promised.  Please help.