Charging me more than what I agreed to

Good day, I did see that I had calls I missed. Unfortunately I am struggling to get my call answered dialling 111. The missed calls was only in the morning. I am unavailable in the mornings. However since I received another sms on another line of mine that I will pay R109 and the contract was signed for R79 per month as a 24 month promo deal. So it has not been 24 months. What is the point of a contract actually? All my lines are going up. All my contracts are taken on what I can afford but as I heard before I can not do anything about it...I feel deals should not be made for cheap and then 5 months or so later pushed up. Please contact me during the hours I am available which is any time from 2PM. I know I might not have the most contracts but I am still a customer. These deals should not be advertised at a certain price for 24 months it should be clearly stated that the price range is different when ever Vodacom feels like it. Regards. Jacqui.