Bill is sent to collection even the device is returned

I've had a nightmare with T-Mobile! I sent the booster back because the new phone I had just purchased didn't work. I told them I wanted to cancel the service with the booster. They never cancelled it. I am still using YouTube tv. They say they never received the booster. I don't have the receipt. They keep billing me for it. After I purchased my new Apple iPhone 13, I couldn't receive or make phone calls. I spent 16 hours over 3 days texting with them to get the phone to work. I spoke to 10 different people over the 3 days. Again, I had already told them I wanted to cancel. In my opinion. this is totally unacceptable. I wish I had never purchased the phone because it doesn't have the reception my XR phone did. It drops calls all the time. I always could talk on my dock or boat but not with this phone. I will contact BBB. I want my money back. I have several attorney friends that said they will help me. They agreed with me that this is totally unacceptable. Any suggestions?