MTN is getting very useless and doesnt treat their loyal customers faily

I have been a customer of MTN for more than 20 years and lately they keep on treating me like trash with their substandard service. I recently upgraded my phone and I asked them to confirm if there will be any fees applicable on the upgrade. They assured me that they will be no upgrade fees. I have just received my first invoice and I see that they have charged me over R2500 for the upgrade. I cancel some of my contracts last year due to their poor customer care. I seriously regret giving them another chance. Please urgently reverse the amount that you have billed me incorrectly. I think I have learnt my lesson that MTN is useless and this will definitely be my last contract. You have disappointed me big time and you keep on doing it. This is not how you should treat clients that are loyal to your brand.