Why is paying off my device so hard?

After a horrible experience, with xfinity, mobile  (broken promises and extremely rude arrogant and unintelligent customer service agents and my free device costing me $16. and change monthy) I canceled all of my xfinity services. I traded my phone into another provider and they issued me a virtual credit card to pay the remaining balance on free phone. I called in to payoff a device 2 hours of transfers and being hung up on and 9 representatives later this is still not done.  The lack of customer service and the inability of your company to fulfill a simple request is why I will NEVER pay another penny to your company.