How do you escalate a billing issue?

I briefly had a wireless account with my partner, which we ended up canceling we never received part of our order after 3 weeks.

We returned the phone received and closed out the account, and even got refunded for the phone we didn't receive the returned phone my husband used.

Now, we're in collections from AT&T for a $1000 bill that I was told was a mistake on AT&T's part?! 

When I first received the bill, I called the support line and they told me there was an error with their billing system and that I could ignore it as they were working to get it resolved. I took their word until I got a notice that AT&T sent our bill to collections for us refusing to pay up. 

I called the support team again, gave them my account information, even the phone's serial number so that they could look up and confirm that it had been returned and that we didn't owe anything anymore. Only after giving them my account information, confirming it was a mobile account, having them look up the serial number to confirm that it was received, they informed me that I was talking to the wrong support team and that I needed to be transferred to the wireless support team. I called the number listed on the website for wireless support services?! Apparently, the best number for wireless issues is 1-800-947-5096 - not the numbers listed here at https://www.att.com/support/contact-us/wireless/

After being put through 5 different support team members and staying on the phone with them for an hour, they transferred me to a closed support office. 

I'm trying to figure out how to get this resolved and how to get it escalated, because they have given me the run-around since day one and I extremely frustrated.

***** I'm sure that I'm not the only one that's had this issue, seeing as the first support team member said it was a billing issue that gave a bunch of customers incorrect/overcharged bills?

[Edited to comply with Guidelines]