Unhappy customer

I've been with MTN for years and I have never been with another service provider. How can the company opt you in for subscription you never signed up for. I have purchased R209 airtime yesterday only to find out later in the day that i am only left with R160. When I called and spoke to an agent Sinenhlanhla Mkhabela she told me that it is due to a subscription which I never signed up for. I am not happy with the service and customer support that is not very helpful because they can't even tell me how I was opted in for the subscription.