According to SA contract law, Afrihost.
If you're referring to this specific case though, OP is contracted to both MTN and Afrihost.
I see the further explanations show that this is an MTN billing problem, but certainly product-specific as well. It's an important customer service issue to address in business plans. If the answer to this sort of issue is "not my problem" then you're simply opening yourself up to serious complaints, as evidenced by this thread. An average customer is not privy to supplier-level agreements and intricate billing procedures.
While I accept that the decision is that of MTN's, there should be a formal policy of addressing such issues when they arise, and facilitation should take place via the product owner - Afrihost. Such issues arise directly due to their product and do have a business impact. It's silly not to prepare for such an incident and to throw the "not my problem" excuse around. Customer service is often misconstrued by companies to be a blame-game when in fact it is about resolution. If you don't prepare for successful resolution of a foreseeable issue by instituting proper procedures for managing such an eventuality, then you're not preparing properly imho.
This was foreseeable. This does impact on Afrihost's mobile offering directly. Customers contract with Afrihost, and such issues arise due to the product itself. Afrihost should have better policies in place than "not my problem" and it should be easily communicated to customers. Had there been, this issue wouldn't have made it to mybroadband.
And Afriman, I don't intend this as an Afrihost-bashing - it's intended as constructive feedback. Covering the cost of a confirmed supplier screw-up is a daily course of business. There should be mechanisms in place for Afrihost and MTN to negotiate behind the scenes with repayment from one party to the other based on requirements that the two companies have in place. The cash flow knock however should be that of Afrihost's and it should never reach a point where the customer is forced to have knowledge of your systems and processes, perform the detective work themselves, effectively prove innocence in the process and then negotiate with a separate entity for reimbursement. It's incredibly simple to contract this with MTN between the two companies; it sets a great example; reflects well on the brand and the product; simplifies customer service; and is measurable in terms of brand benefit. I'm not suggesting a blanket-policy of reimbursement, but rather one that's based on sound business principles and agreements, instead of dicking a customer around. Your financial exposure here is balanced, if not outweighed by the brand value gained. I could probably perform the calculations for you if you'd like, but I charge for that...