Why was one of our four iPhone trade-ins "rejected"?

Phobio is processing trade-ins for Apple, at least here in the US. I took it to mean Apple would not issue them a "certification of erasure" when they tried to "erase the data from the device". The phone was bought used and had no issues on T-Mobile for the past 4 years. All four phones were factory reset before shipping to Phobio and all showed the Hello in different languages when powered on.

The part that I find disturbing is the "help" I got from my Phobio chat was I should "contact Apple to see what option they have to resolve this issue." Isn't that something Apple and Phobio should have worked out long ago? Surely our phone was not the first to be denied a certification of erasure by Apple. I don't even know who to contact at Apple about this, which is why I started here.

I clicked through "restore device to factory settings" and it said someone from Apple Support would call me within 2 minutes. But I don't have the phone in hand. I rejected Phobio's trade-in offer of $0.00, so they will be returning the phone to me in a few days. I'll talk to them then since they'll probably need some info from the phone. Thanks for your help!