These three prepaid GSM providers will not allow you service unless you have a bank account which you must use for the initial payment before activation:

• Mobile Vikings
• JIM Mobile (same ownership as Mobile Vikings)
• Scarlet
• (edit) Ello? They might have the same issue as the above three

At the same time, some banks will not allow you to open an account unless you provide to them a mobile phone number registered in your name with proof of that registration.

You open a “basic” bank account at a bank that offers those kind of accounts just for the one-time purpose of getting a sim chip from one of the 3 MVNOs, but Belgium has a separate rule that blocks basic accounts from receiving cash, even a small amount like €10.

So you must obtain a sim from a mobile carrier other than the three to get a normal bank account open which accepts cash. Then use that bank account to buy the Jim or Scarlet sim card. Then credit is trapped on the 1st sim card. You can do a phone number transfer to get it credited back, after they siphon off €5 for the porting effort.