Unaware of subscription

I took a contact laptop with MTN last year April, that comes with a sim card offering 2GB per month. Today I bought R160 airtime and when viewing my balance it came to my surprise that I only had R40 airtime, I called the call center, and I was advised that I should dial the *155# string to check if am not on any subscription, which I did, only to find out that I am on some subscription that takes R20 airtime everyday from my account. bear you that I've never used the sim card for anything besides hotspot. So I just don't understand how I can be on a subscription which I have never subscribed for.