Carrier Direct Billing is a charging method that works via your Telephony provider (Vodafone, Three, Singtel etc) and applies to digital services. This way of paying for content is very convenient, because you don’t have to send a SMS or anything else – just click on the payment button (or subscribe button, depending on the service), confirm your purchase and the charges will go onto your phone bill.
After clicking to accept the costs, in most cases you will receive a free SMS confirming the event and, in case of a subscription, the SMS will be detailing the costs of the subscription, how to end the subscription and a help desk number.
Nowadays, most Telecommunication Carriers use a double opt-in mechanism, especially where it comes to subscriptions. In some countries, single click one-off purchases are allowed.