Once you arrive in Brussels, you probably will want to have a Belgian phone number, in order to connect with the other trainees and not having to pay ridiculous roaming rates. There are many mobile companies you can choose from, but, as I haven't done a profound marketing research on them and just chose the first which I found interesting, I'll only tell you about that one: Mobile Viking!
This is an online mobile company, the website is all written in English and is quite self explanatory. I chose it because all other trainees were mentioning it and sharing good experiences about this company, so I looked at the rates and they seemed great (you can have internet, free sms and 1h per day on calls to other vikings for just 15€ per month), so I decided to give it a try.
I ordered a SIM card through the website and it arrived within 3 days, along with a letter with instructions on how to proceed next. I then activated it on the website, I "topped up" with 15€ and it is all working fine so far. After one month, my communications will expire and I'll have to top up again. It seems a great option for those who own a smartphone and want to have internet in it... along with the sms offer (1000 free sms for non-vikings and 1000 free sms for vikings) and the 1h per day call limit (just for vikings and limited to the first 200.000 registrations, which are still far from being reached), is a great option and all I'll need during my stay here. I like to have a fixed rate that includes a lot of things so that I'll know for sure how much I'm going to spend on communications each month, so this definitely does that for me.
If you decide to choose Viking as well, you have the opportunity of referencing another viking number during the registration process and that person will be awarded some points (which can then be exchanged for credit), so if you were influenciated by this little piece of information and thought it helped making up your mind, feel free to use my number on this: 486304922. Thanks!