Voice markers have a notorious cross-linguistic tendency towards multifunctionality, in that a given marker can encode more than one voice operation at a time, such as reflexive and passive. In addition, diachronic typological research has also shown that patterns of multifunctionality of voice markers historically come about following paths that are not necessarily unidirectional. Taking stock of these premises, in this paper we propose a new typology of voice markers grounded on the notion of underspecification and ambiguity, and, by adopting the perspective of Functional Discourse Grammar, we argue that the lack of unidirectionality in the grammaticalization of voice markers follows from their status as interface operators.