Skewed billing, no response from customer services

MTN has been billing me over and above the R550 top-up option I had agreed to during the Corporate Sales call I made assisted by their agent Lungi (***) to upgrade from a Samsung S3 to a Samsung S6 device in May 2015. Statements reflect that the new billing started in May and yet the device was only delivered to me at the end of July after I asked Lungi to cancel the upgrade as they were failing to deliver the device for almost two months. Her superior begged me accept the late delivery and I could top up with airtime and data bundles in addition to the agreed R550 each month from receiving the device in July. MTN has billed me exorbitant amounts and I raised this with Fazila Mustapa at the Pavilion Durban branch of MTN who sent an email to customer services on 27 September 2015 and said I would receive a call from MTN within 24 to 48 hours. It is now November and no word but billing from MTN. Please trace *** from Yogan of the same branch about two weeks ago. I have received no resolution to this matter whatsoever. I feel helpless.