New methods for speaker verification that address the problems of limited training data and unknown telephone channel are presented. We describe a system for studying the feasibility of telephone based voice signatures for electronic documents that uses speaker verification with a fixed test phrase but very limited data for training speaker models. We examine three methods for speaker verification that address these characteristics in different ways, including text- independent mixture models, a broad phonetic category model that has some of the properties of both text-dependent and text-independent approaches, and a text-dependent approach based on speaker adaptation. The speaker-adaptive approach is shown to have significantly better performance when the training and test channel conditions are mismatched, resulting in better overall performance across all conditions.