Vodacom contract rate increased despite reduced subscription agreed previously

I have been a Vodacom subscriber since 2004. When my latest upgrade was due, I was called and agreed to package that would cost me R305 per month. When I received my first bill after agreeing the new contract, the subscription was around R385. This is around a 25% increase on what I had agreed upon. I called Vodacom to query and was told that they had increased the rates for this package. How is that acceptable if I signed a contract with them for R305. Are Vodacom allowed to agree a contract for a certain value and then just increase without and reason. Can I please have some feedback for this.