MTN: It doesn't add up

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I just love getting statements from my cellphone company, MTN. They are just full of laughs. I have noticed that for some time my debit order was R89 but the balance shown is double this amount. Looking back to figure out why, I am still completely confused. In November last year it just increased by R89 with no explanation given.
The call centre operators just repeat the mantra "it's because we invoice monthly in arrears" and cannot explain any further. I guess it's not in the script and they are just underpaid droids. At least the phone works, even if they can't add. Such is life in Africa.

Update 22nd June 2011: After meeting with several MTN executives they helpfully sent me a reconciliation spreadsheet going back several years. I then discovered the following:
I am therefore left with the conclusion that all statements between Feb 2009 and Nov 2010 were incorrect, and the bug has now been fixed. Weird.