Best Local SIM Cards for Data and Calls (Brussels)

Dear All,
I have a client that travels regularly to Belgium, and in order to avoid roaming charges, we are looking for the best SIM card only plan that would include data and calls, like prepaid/pay as you go.

We are not looking for global roaming card, just the regular local networks. So far I have only found Mobile Vikings

Is this common practice, can you just go and buy them freely? You can do this in the UK, but I was quite surprised to find that in Germany you need proof of address, even if it is a prepaid card.

Thanks for any input,

Ana Clara