Random debit orders from Telkom on cellphone contract with no explanation

I have a cellphone contract with Telkom (number ***) and was notified that I was due for an upgrade. I went to the IStore at the V and A Waterfront in Cape Town on the 18th of January 2025 to do the upgrade and signed for a contract costing R 689 per month. The first debit order went through at R710.00 instead of R689; for the second debit order (6th March) I was charged R1286. I called customer care and spoke to Mr Thabo Mapoepa (6th March, 9h08) and was told that the overcharge was due to having to still pay for the device from the previous contract. The explanation did not make sense to me as I received messages notifying that I was due for an upgrade and when I went to the Istore, they confirmed that I qualified for the upgrade. Additionally, the billing has been random, with the first debit order amounting to R 710 and the second debit order to R 1286. Mr Thabo was not able to explain what happened. I feel this is ***** as Telkom is randomly charging my account without an explanation and customer care is absolutely helpless. I hope someone will be able to assist, Stop Telkom from taking money from my account and reverse the portion of the payments wrongfully charged. Absolutely shocking!