Unwarranted billing

My wife was offered a gadget - a travel Wi-Fi router of sorts. She accepted the offer on the basis that she was NOT to be given an additional, or different number. She was assured that no additional numbers would be provided. When the unit arrived, we noticed a new number had indeed been provided. We had not yet opened the box. We called, and to sum up the exchange, were told we could return the unit. Which we duly did. Vodacom have it on record that the unit was returned on 25 January 2015. Vodacom have however continued to bill us for this unit, and despite visits to Vodacom The Bridge