Hi PhilaniH,
In Agreement with Bigx333, any voip provider should have a maximum balance / prepaid limit assigned for each client. In fact, this is mandatory for the majority of systems and generally defaulted to a maximum of R 5000.
I would recommend requesting a detailed call report, with a copy of the agreement signed with the service provider. If the call amount genuinely amounts to R 10 000, I would recommend disputing the invoice, and demanding a significant discount, as your billing was allowed to rise way beyond reasonable usage due to the voip provider's neglect in setting billing thresholds.
I know of a large South African ISP who recently billed a particular client R 135 000 for out of bundle roaming data usage, but the amount was reduced to R 14 000 after he disputed it, claiming negligence by the ISP for allowing his bill to raise so high without warning or notification.
Good luck!!