Actually all these occur is when you actually unknowingly or
accidentally sign-up for all this premium services. The companies
like Zingmobile, send unsolicited messages from any number they
grab. If you respond, they take it as you want to sign up for it.
Thats why, when you call the Telco like starhub, they ask you to
contact the service provider yourself as the money is not earned by
Starhub but by the other companies like Zingmobile.
My 2cents from my exp working at a Telco. I've seen bill going up
to a few hundred dollars.
