Charging more money on contract phones

I have a cellphone contract with Vodacom and they're charging me more than what we agreed on every month the amount goes up, the initial payment was 650 I've paid 850 last month this month the email came and said I must pay 1250, tried to get some explanation from the store at Cresta mall but they couldn't help, so please people don't ever think about getting a contact at Vodacom I am warning you.