Terrible ATT customer service

New customer that switched 7 lines from verizon and I am now looking at a $1,563 incorrect charge in a few days. In short....

1. Brand new phone and brand new customer, phone was a lemon and didn’t work. Called att and although I was within the 30 day warranty a new phone was shipped to me as an “upgrade”.

2. I immediately returned the broken phone and even have a tracking number from usps that it was returned to them.

3. Despite many many calls to customer service, everyone agrees that this is a mistake but no one knows to fix this. All I get is that we will be taking care of this and emailing you. So I get the first email but all subsequent emails are ignored. 

I'm at my wits end here and severely regretting the switch from Verizon. Anyone have any ideas?