Abstract: The invention relates to a method and system for authorizing a commercial transaction between a customer and a goods or service provider via an authorization service provider with authentication of the customer by comparing a biometric sample of the customer with a biometric model of the customer stored with the authorization service provider. The telephone link between the supplier and the authorization service provider is set up via a router which associates the real telephone number of the authorization service provider and the telephone number of the supplier with a virtual telephone number and transmits a code which is isochronously calculated by the router and the authorization provider with the aid of the same algorithm.