What the hell?

I keep getting the following sms. \Your recurring R7.00 Subscription to AirG Social Service will be billed on 2015-06-07. To unsubscribe click [URL Removed] :)". I clicked on unsubscribe and found I had a 1 week free trial on my accound after which I would be billed R7 per week!!! I have never in my life heard of airg nor subscribed to anything via my phone. How does this happen vodacom? I am a teacher and I really don't need big corporate companies taking away money I barely have. So how does it happen that I can subscribe to a service I have never heard of? I am a highly suspicious customer at the moment."""