This experiment investigated the effect of interface metaphor and context of use (private/public) on the usability of a hierarchically structured speech-activated mobile city guide service. Two different versions of the service were evaluated using a Wizard of Oz methodology. The first was a non-metaphor standard service with numbered menu options. The second was a service based on an office filing system metaphor, with different metaphor-related menu options at each level. User performance and attitudes to the services were recorded over a six week period, and post-trial interviews conducted. Results showed that the interface metaphor improved participants' performance compared to the standard service, but had no effect on attitudes. Context of use did not affect the usability of the services, which supports their use for mobile interaction. Visualisation of the metaphor-based service significantly affected participants' attitudes, suggesting an additional benefit of using interface metaphor for the design of speech-based mobile phone services.