Verizon owe me $300 and I contact them five times still didn't get it


What should I do? I contacted verizon customer service like five times. They owe me $300 dollars on my iphone upgrade more than a year ago. They keep putting in claims for me but it never go anywhere. I have spend hours trying to get a agent to issue me refund. 

It's a promotion for bring your own device. They are suppose to issue a discount of $300 to my account. And I haven't see anything after more than a year. 

How could a customer service be this inefficient? I feel like Verizon is trying to take advantage of me. 

What should I do? Thank you!