In a network in which to set up a call to a called endpoint a calling endpoint sends (4) the called endpoint a call set-up message including a source information element identifying the calling endpoint and a destination information element identifying the called endpoint, the call is set up on condition that the call set-up message contains a first token value (J) that corresponds to a second token value given by an equation that includes said source information element, said destination information element, and a random information element generated by the network to be associated secretly with the first token value in the network.