This paper describes research work into the design, implementation and usability evaluation of intelligent dialogues for automated telephone services. The project involves a series of large-scale field experiments using a new real-time Wizard of Oz (WOZ) scheme designed to permit investigation of users' attitudes towards simulated automated telephone services, and the evaluation of users' perceived usability of such services. The project is distinguished from most previous studies by the choice of highly constrained application domains; by the degree of control the WOZ software provides over the experimental variables; by the care being taken to quantitatively measure users' attitudes and perceived usability; and by the large subject population (1000 UK subscribers) available for use in the experiments. The experimental programme itself consists of a series of WOZ experiments in which the characteristics of the speech interface (including an automatic speech recogniser) are manipulated and the dependent variables of user attitude and perceived usability are measured by both a telephone questionnaire and a postal questionnaire. Experimental results with a subject population of 256 subscribers are discussed in the paper.