Black listed because I’m switching carriers! Shame!

I switched carriers after many years with Xfinity. I can’t turn my phone in for trade in with the new carrier because Xfinity has black listed it. This is sneaky and shady business practices. I’ve spent hours on the phone, over 7 separate calls and they don’t fix it. They claim to fix it but it’s still blacklisted. This is ridiculous. I’ve paid every bill on time and the phone’s been paid off for over 2 years. Shame on you Xfinity. You need to fix this and do better.