We discuss the potential to observe lepton number violation in displaced vertex searches for heavy neutral leptons at future lepton colliders. Even though a direct detection of lepton number violation is impossible for the dominant production channel because lepton number is carried away by an unobservable neutrino, there are several signatures of lepton number violation that can be searched for. They include the angular distribution and spectrum of decay products as well as the heavy neutral lepton lifetime. We comment on the perspectives to observe lepton number violation in realistic neutrino mass models and argue that the dichotomy of Dirac vs Majorana heavy neutral leptons is in general not sufficient to effectively capture their phenomenology, but these extreme cases nevertheless represent well-defined benchmarks for experimental searches. Finally, we present accurate analytic estimates for the number of events and sensitivity regions during the Z-pole run for both Majorana and Dirac heavy neutral leptons.