An authentication server and a method of enabling a user of a communications capable device to make a payment via a mobile operator billing system are described. The authentication server determines a user identifier string for sending to a communications capable device according to a probability of successful entry by a user. The user identifier string is linked to the content item data and the string sent to the communications capable device. The user responds, via a mobile network, by sending a short message service (SMS) based user response. The response is compared to previously transmitted user identifier messages, and billable user identity data appended to the SMS by the mobile network can be used to associate the transaction in process with billable identity data derived from the carrier data to enable a user's mobile phone account to be billed for the purchase.