disgusting service

I signed up with vodacom and was told because I was a new customer I would receive a R 300 Mr D voucher. When I received the voucher Mr D told me it had expired 6 months before I opened my vodacom contract. I called vodacom who said it would be resolved in 7 days. I have been calling vodacom for 8 weeks to resolve this and they ignore all my calls and emails.