incompetence

I settled one of my contracts in Feb this year. I paid it at the vodashop in CW.. and thought I was done. Then at the end of may they debited me over R4000 and when I contacted then several of their people said it was a mistake..but not before I was transferred from one dept to the other and hung up on many times. I spoke to a manager who said he will investigate and get back to me and still hasn't. They advised they never processed the settlement and I owe them R2500 because the money I paid was used for the bill for Feb and match. . How is the my issue. I refuse to pay that money as I can't afford it. I pay Vodacom in excess of almost 20k a year and I have been with them for over 8yrs. It's been 2 months and no one called me back. I'm am appalled and will take this to the ombudsman. Someone changed how I receive my bills and sent it to an inactive email address when I didn't ask then to do it. I believe they need to s**** that money as it's their delay and incompetence that has caused this. When I asked for the settlement amount I asked if I need to call them when it's paid& they said no. It will be processed within 24 hours of the money being received. But they **** I'm fed up