I have a similar situation with a Cell C SIM in a remotely located GSM alarm system.
Cell C needs to add additional functionality to their/there

website to allow a voucher recharge and data bundle purchase, much like you are able to do on the Vodacom website.
8ta's website is functionally great. It may not be fancy, but it is brilliantly simple, as your account balance, redeem airtime voucher and purchase bundle are all on the same page and not hidden amongst other menu options.
As far as I know, you cannot even send airtime from one Cell C number to another.
....time for Cell C to catch up......