Waiting on BOGO Credit From December

AT&T, I am angry & frustrated. In December, I signed up for service on 4 new lines and qualified for a holiday BOGO phone promotion on 1 of my phones on my Next Plan. It is now September, and I am paying installment 10 of 24 for this phone ($24.38/month). Since April, I have exhausted ever single line of customer service communication available. I have called customer service, chatted with online via your website at least twice a month, and even contacted the Social Media Team. Each of these methods of communication require at least an hour of my time and I have gotten nowhere. Everyone agrees that I should be getting the credits, but each month a bill is issued and I never receive the credit.  I was also promised on 5 occasions that managers and people from joint billing would call me back to resolve this issue. I'm still waiting for those calls. I am not asking for anything that wasn't in my original promotion. 

Unfortunately my next step is going to be to cancel my service. I just want this resolved.