  We discuss the potential to observe lepton number violation (LNV) in
displaced vertex searches for heavy neutral leptons (HNLs) at future lepton
colliders. Even though a direct detection of LNV is impossible for the dominant
production channel because lepton number is carried away by an unobservable
neutrino, there are several signatures of LNV that can be searched for. They
include the angular distribution and spectrum of decay products as well as the
HNL lifetime. We comment on the perspectives to observe LNV in realistic
neutrino mass models and argue that the dichotomy of Dirac vs Majorana HNLs 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 HNLs.
