contract

last year i took contract with vodacom with the agreement of paying R200 per month on 26th and they have being debiting more than R200 as agreed and they have being on the 25th and didnt have problem with that but now June i tried to debit on the 25th and there were no funds as am getting paid on the 26 and on the 26 they debited R300 and then i returned the money because now am tried with debiting the amount we did agree on. and for July they did not load airtime as they suppose to and now they are busy calling me to pay amount of R681 because of the June payment and July. and my question why should pay 4 something i didnt get. Am really fedup ke VODACOM KE MA....PA FELA.