Worst AT&T business service customer center

AT&T's service is truly the worst I have ever experienced. I closed my business internet service two months ago and returned the modem through UPS immediately. I even received a notification email from AT&T confirming they had received it. According to the final bill, there is a credit amount that I should receive, but I still haven't received it. Every week, I call customer service, and they give me a case number and tell me to contact the refund center, promising they will send it, but I still haven't received it. They even said it hasn't been sent yet. Every time I call, I'm never connected right away and I'm constantly told to call different departments. I've wasted five hours of my time dealing with this. I will never use AT&T Business internet services again in my life.