It's not true. Etf is tied to your account. Blacklisting a imei is for lost or stolen. If u default, it's considered a "bad" esn cuz there's an account with a balance on the phone, not cuz AT&T blacklisted it.
If you don't pay your bill, the bill and etf is on the phone the account is on.
Again u can buy a iPhone on a 2 year contract, swap esns, and that iPhone will have a clean esn. If u stop paying or cancel, the account is to the phone the account is on. So is the etf. It's all about the line and service.
It's no different if u buy a clean esn phone off eBay and do a esn swap. Once u do that the account and etf is on the new phone. The old phone is clean.
Just like carrier insurance like asurion. They want your damaged phone so that when you swap your account to the replacement, they repair it, turn around and sell it cuz it's got a clean esn on it.
I know we're going back and forth here lol. Just saying I know from being a dealer this works. We do postpaid, prepaid, phone flashing, etc. we work with CDMA and gsm phones. Verizon and AT&T, along with sprint & tmobile.