Cannot add top up airtime on contract

I recently took out a data contract with Vodacom and was assured that I will be able to add a recurring top up of airtime on the sim and this will be reflected on my monthly bill. I took out the contract and when I queried with Vodacom I was told that because it was data sim no airtime could be added on a re-curring basis. I can however add this monthly but the payment is via credit card which is not ideal.