A method for making a purchase of products or services via the Internet by a customer from a vendor includes transmitting, by the customer, a purchase request and mobile subscriber identification data representing the mobile subscriber identification number to the vendor via a mobile radio network. The vendor forwards the purchase request and the mobile subscriber identification data to a security device. A transaction number for the purchase present in the mobile radio network is then transmitted to the security device and the customer. The customer sends the transaction number it receives to the vendor and this transaction number received by the vendor is checked for validity. If the transaction number is valid, billing for the purchase is initiated.