At present, you're not yet required to identify yourself. But you might as well already do so voluntarily as it will become compulsory very soon.
The only thing you have to do is surf to www.proximus.be/identify with your identity card, PIN code and an e-ID reader. You can also identify yourself by topping up your Pay&Go card on this same link, using your Bancontact bank card. Or ask a member of your family to do this for you.
Don't have Internet access? In that case you can always drop into a Proximus Center near you.