Paying for an account I cancelled

I had a cellphone contract with mtn which was meant to end in february 2017. On the 8th of February 2017 I wrote a letter of cancellation to MTN and requested to keep my number. On the 2nd of September 2017 I received an sms from MTN sp informing me that my had been handed over to VVM debt collectors. When I went to inquire at the MTN sp in Vaal Mall vanderbijlpark, I was told I owe MTN R3000 that I had to pay for my name to be removed from the list of bad payers. Even though I didn't owe them a dime at the time I handed my letter of cancellation in February 2017, I still paid them R3000 on the 11th of November 2017 in order to clear my name. Upon further enquiries made by myself, I was then promised a refund of R2900 based on the fact that the error was not mine but theirs. I don't know why they didn't offer to repay me all my money. It's been a over a month now and I'm still waiting. I've wasted my petrol, airtime, time and energy following up on this issue. Frankly, I'm just fed up.