Article 101 of the French Code of Criminal Procedure
The examining magistrate shall summon before him, by a bailiff or a law enforcement officer, all persons whose testimony he deems useful. A copy of this summons shall be delivered to them. Witnesses may also be summoned by simple letter, by registered letter or through administrative channels; they may also appear voluntarily. When summoned or summoned, witnesses are advised that, if they do not appear or refuse to appear, they…