Invalid Gift Card

I received a Homemark gift voucher from Afrizon Telecommunications along with my Vodacom contract cellphone and SIM card. I tried to phone Afrizon to activate the gift card, but no one answers the phone so I sent an email with my personal particulars and the gift card number asking them to activate the gift card. I received an email prompting me to wait for the SMS that notifies me that the gift card has been activated. I received the SMS notifying me that the gift card has been activated, but I tried purchasing from Homemark, but according to them the gift card is invalid!