Seems its the billing system that does not allow this. If I wanted a recurring data bundle, I could activate via ussd. However a once off data bundle is not treated the same, reason being a recurring data bundle is charged for upfront, on a month to month basis, and a prorata amount is billed in the initial month of activation. A once-off data bundle would have to be billed to the account afterwards. The billing system does not allow for this, however with voice its treated differently, I would be billed in the following month for any calls over and above my initial R50 allocation up to a credit limit amount of R1200.00
What gets me is that from time to time I can't buy extra data on an ad-hoc once-off basis unless I call customer service and ask them to set it up and give me the go-ahead, and if I want to cancel a recurring data bundle that I initiated via ussd I now can't action via ussd, I have to call customer care.