Pathetic service

I did an early uodrade on 20 December 2021.. I was not happy with the device and returned the device within the cooling off period before my next billing told.. I was told that I won't be billed the early uograde fee as it has been reversed.. I have just received a bill for bldoubke the amount that I am paying. I spoke to numerous people no can assist me. Getting transferred from person to person and they have no track of previous conversation I had every single day since I took the device back.. They are rude and cold transfer calls.. I have never experienced such bad service.. I'm now being billed for something I don't even have