Phone upgrade

Maybe this community can explain the upgrade. If I turned in a phone and received the full upgrade amount - why does my bill still show I owe the original price for the new phone purchased? 
I was supposed to have been credited 300 but it is not reflected on my bill. I feel like I'm being cheated out of my money and unfotunately i'm stuck. 

Re: Phone upgrade
Customer Service Rep

Hello, there! We understand that you are concerned you will not get the $300 credited for the phone you purchased. You should see the credits start on the 2nd or 3rd billing cycle, and when they do start, you will receive all the missed credits on that bill. If you do not see these credits by the third billing cycle, please contact us again, and we will manage that for you, manually.