billing for item i returned

I upgraded last year in October however I returned the cellphone and took a laptop instead. Vodacom has been billing me for the cellphone for 8 months and I've been calling to find out why but they state that I was still paying for my old handset. I went to Vodacom store at the workshop where it was discovered that I was paying for a cellphone that I do not have. Its so disappointing because Vodacom is such a big company but are slipping up on the basics