T Mobile Warning

I hate corrupt and under-handed business dealings.
Popped into t mobile for a temp phone to see if service was better in my area. It was not. Took the phone back the next day. 1 call. 1 text message. Less than 100 mb used to run a few speed tests.
Guy hands me a receipt. Tells me it is taken care of.
Couple months go by.
I get a call from a collections agency. I explain the situation. I do not have a T mobile account. He sounds like this has happened before. Connects me with t mobile. They tell me go to the store.
I find out the guy who handled my account has been canned. This was done to a number of people.
They call the uppers. They reply the account was never actually closed. Therefore, there is nothing they can do. So the guy has to call someone higher up. They assure me it will be dealt with but it may take a few days.
So I am now awaiting a call.
At this point, I went digging through my junk mail where I found an UNMARKED envelope that contained a statement from t mobile about this matter.
So my conclusion is this:
Did the guy get kickback?
Did they send the notice in an unmarked envelope hoping I would throw it out thinking it was junk mail?
Did they sell my bill to the debt collector?
If so, then was that the plan all along?
They have made a profit off of services that were never rendered and have put a black mark on my otherwise good credit.
This seems to be a habit with t mobile
If you cancel a plan with tmobile, do not leave the store until you have definite proof of cancellation.