Vodacom increasing contract prices

I received 2 emails w.r.t. my 2 current contracts at Vodacom indicating price increases. I understood this to be fixed for the 2 years that you signed it for. They have never increased their prices in the middle of a contract. <br> <br> Can they just do this when you signed up for a specific amount for a 2 year period?? This doesn't sound right.<br> <br>