Who I have the same questioned this topic

It has been over a year since I switched from AT&T and took advantage of a promotion where I would receive $300 trade-in credit for my old iphone 6. According to my bill, I was only credited $100. I made at least 3 calls to customer service, and each time they said it would be fixed and to wait a couple months before I saw it on my bill. I just recently called again because the credit has not appeared and the customer service person basically told me I was out of luck and they are not honoring the promotion. This is terrible customer service and I'm extremely disappointed. I am definitely reconsidering changing carriers when this current contract is up.