overcharged

Vodacom has been *****ing me I took a contract in January via call centre for premium of 249 then the first month I pay more than 300 the second month was the same then I call the call centre they told me im paying for handset till end of march. then on 16 April I call a centre again the agent told me that who ever sold me was wrong the total im suppose to pay is 328. my question was whose fault was it because I was told that I will pay 249 then they promise to call which they've been doing and I know they wont. so i'm not sure what I must do I thing of not paying until they fix that problem because I cant afford 300.