Pathetic service provider

I took out a contract just over a year ago that has a monthly fixed subscription. Today I received a sms about the amount due and it is more expensive although the amount was supposed to be locked in for 24 months. I contacted the call centre about this and the consultant could not explain why the increase. She communicated that her supervisor will come to me and I am still waiting. I have not added any additional things to the contract. This is not rocket science….surely it is easy to pick up that it is an error and why. They need to submit proof to me why the increase but knowing them…I can only wonder. **** service from them.