Increase on monthly debit order

MTN have increased my monthly debit order without notifying me . When I called them , they mentioned all customers was notified , when I request a proof they did not provide. Accordingly to them they have the power to that. Surely it's my money and I signed a contract for 2 years with a fixed amount. When I request to talk to management no reply. I am a customer over 30 years with them this never happened before. This can't be right that creditors have the right to change an amount of debit order without authorization. What will happened to the customer who can't afford the increase. Why must the customer pay for " unforeseen" cost , bad management etc.Regards