Like Cambodia, there are a number of telecom operators available to offer a SIM Card. However, most of them require the Belgian Residence Permit, which you might receive weeks after your arrival.
BASE is the only telecom operator which only needs your passport when you buy a SIM Card from their shop. Personally, I find BASE kind of satisfactory to use and never want to change to another telecom operator. Please check out the BASE's location in Hasselt (https://goo.gl/maps/YLfWxat3BrdBVjcWA) and their official website (https://www.base.be/en/private/our-offer/prepaid-cards.html).
In addition, when you visit any EUROPEAN member states, BASE still charges at the same rate, which is so comfortable in my opinion.
