Im tired of you billing me

My contract expired on the 23rd of May 2015 and I went to Telkom shop in Clearwater to cancel it and convert it to prepaid. I was told its gonna take a month for this changes to reflect. For duration of a contract I was paying R549 per month. June 2015 I was billed R995 (why double now because my contract expired?) I called call centre and was told my case would be passed to billing department coz the lady couldn't understand why this was happening. July 2015 I was billed R1500 still no consistency in this billing method (Why??) I called again couldn't get explanation they sent my case to billing section I didn't receive response. I went to branch still no help. Now I was told I will be debit R2000 coz my account is on arrears (Still no consistency, why??),today I called billing and they said I should wait they are retrieving my file for 25 minutes after that they decide to close the call. I need answers.