data and airtime

i have a prepaid number <br> very Unhappy with vodacom service as it finished my airtime once the data is completed , this is the third time happening as i lost 300 rand's airtime. i did buy a data bundle separately but when it is finished it should t use airtime without my knowledge. it is know as one of the best cellular provider and it happens like this. did call the customer care they said it is like that that's the answer they gave me. useless service.