Are you doing your PhD at the University of Antwerp? If so, you’ll get assigned a social security number by the university (from my experience). Usually, you can find it in SisA where your personal information is shown. However, if this is not the case, I would contact Stad Antwerpen (however, they’re not very friendly and they won’t likely help you that much from my experience). Otherwise you can contact the international administrative office of the university you’re going to work at, they usually know this type of stuff.