1. A method of operating a current dialogue with a user of an interactive voice response system having a dialogue model comprising a plurality of states and a plurality of interconnecting edges; a current state; and user dialogue data indicating for a user a total number of visits to a state; in which a prompt definition, for use by a message generator to generate a message for sending to the user, is selected dependence upon the current state, upon the number of visits made to the current state during the current dialogue and upon the total number of visits said user has made to the current state during one or more previous dialogues.