Current API functionality does not allow users to retrieve all contacts. It retrieves contacts only on lists. It would be great to be able to retrieve either all contacts (including those not on lists) or specifically all contacts not assigned to any lists.
Give the new "include all active contacts" option in segmentation a try. This new option was specifically added to allow you to select active contacts who are not members of any lists.
Vice President - Product and Client Success
Maropost Marketing Cloud