wrong billing

hi another problem with Vodacom<br> i have a vodacom prepaid number and its plan is 79c per min and they have charged me 91 cents for 22 seconds, and some time even more and not working according to the plan. so i did call the customer care and asked about this and the lady named natasha with a very bad attitude saying it is 79 cents per min but it changes, does now how to explain the customer and she kept on saying it increases in the plan and that's it only answer she gave every time. even vodacom is not following according to the plans it told. if this is continues i wouldn't like to continue the vodacom service provider, they may be charging an excess amount on each call. please be careful.