Charging more every month than what was signed for on contract

My husband has been fighting with Vodacom for the last 6 months or more as they changed his monthly amount that he signed for the day he opened his account. He has called at least once a week to try and get this sorted out, he has been into the stores a few times and no one is welling to help him and have it changed. They keep saying they will call him back with an out come and to this day we have had no call from them. They have now blocked his phone due to him refusing to pay the extra amount. These people need to be stopped. I have had enough and from now on we will never open an account with Vodacom again. He have all the proof we need to take this further as Vodacom is now steeling our money