INCREASE

I have signed a contract for the amount of R99.00 - UChoose Flexi. This give me R99.00 airtime per month. I just got a mail that my account is now R110.00 and I have R110.00 airtime per month. Is this acceptable. I thought when signing a contract that what ever is agreed to is the price you will pay for the duration. The extra R11.00 is not the issue but the fact that they can just change what's on a contract. I did not agreed to this and until then I don't think they should be allowed to credit my account according to what they decide or feel like. I think this is a matter for the NCR.