TITLE II COOPERATION WITH THE INTERNATIONAL CRIMINAL COURT
CHAPITRE V OTHER FORMS OF COOPERATION
SECTION I PRINCIPLES
Article 22
Requests for assistance from the Court in relation to an investigation or prosecution must be addressed directly to the central authority.
In accordance with article 93 of the Statute, such requests may include any act that is not prohibited by Belgian law, with a view to facilitating the investigation and prosecution of crimes within the jurisdiction of the Court. They concern in particular:
(5) facilitating the voluntary appearance of persons as witnesses or experts before the Court;
Facilitating voluntary appearance of persons - authority - ICC proceedings
EDIT.