Bad product sales from Telkom

ordered a device through telkom's website in December 2017, was sent an incorrect device in December at the same time, is still being billed for this incorrect device and has never received the correct device. I have sent the device back to telkom in march after numerous calls to Telkom from 3 January 2018 to return the incorrect device and they have still been billing me for this wrong device, Even though I do not possess it anymore.