Unauthorised billing of MTN account

I discovered that for the last 3 months my account was billed up to my set usage limit WITHOUT my authorization. Why/how can this happen? The number given does not exist or the phone cuts of when option to speak to operator is chosen. Why must I contact MTN to stop this? It seems MTN is well aware of this abuse. How can the consumer be protected? I wish to be refunded the full amount for which my account was billed over the last 3 months WITHOUT my authorization.