False advertising

Im am upset at the text message i received from vodacom today, stating that my contract i took out which i signed for at R99 for 2gb is now being increased to R109.<br> How can the pricing change after i have signed a contract for R99 for 24mnths.<br> Does a cell phone contract change once it has been signed?<br> I feel this is false advertising, as the special clearly stated R99p/m and now there's an increase in price!<br>