Vodacom overcharging for SMSs when overseas

I was in Zambia on business in Janaury. On return I received an account where i was charged 5 separate times for sms at a rate of R53.19 to a total of R265.95. the normal charge for a SMS from Zambia is R 1.74. I also have no record of sending the SMS on my phone on the dates indicated ( i have itemised billing). Despite having contacted Vodacom and registered my complaint numerous times since the 30th December, no one there has given me an explanation for this charge. i feel deeply frustrated, they now wont even allow me to register a complaint, i keep getting " this has been completed"