Third generation (3G) services boost mobile multimodal interaction offering users richer communication alternatives for accessing different applications and information services. These 3G services provide more interaction alternatives as well as active learning possibilities than previous technologies but, at the same time, these facts increase the complexity of user interfaces. Therefore, usability in multimodal interfaces has become a key factor in the service design process. In this paper we present the work done to evaluate the usability of automatic video services based on avatars with real potential users of a video-voice mail service. We describe the methodology, the tests carried out and the results and conclusions of the study. This study addresses UMTS/3G problems like the interface model, the voice-image synchronization and the user attention and memory. All the user tests have been carried out using a mobile device to take into account the constraints imposed by the screen size and the presentation and interaction limitations of a current mobile phone.