In previous years, I've always bought a local SIM card as a backup in case I couldn't get onto the wonderful FOSDEM wifi and the SIM has always worked within 5 minutes. I bought one early this morning and it still isn't working yet after about 3 hours. The shop and the customer service line have told me that they have to check all new accounts manually now because of "terrorism" and pressure from French authorities. They told me to wait 24 hours before asking about it again. For anybody only arriving in Brussels today and buying a SIM in the evening, it is not clear if it will really be activated over the weekend or only next week so you may just be wasting time and money. Apparently, you can still buy tobacco products here and consume them immediately and they will eventually kill you but thanks to the irrational fear of Muslims, you can't get immediate access to the Internet in Belgium.