Check with your current provider if you have to pay an early termination fee to cancel your contract (fortunately, this is not usually the case nowadays).
Your number must still be active on the Belgian network of your current provider at the time of transfer. Therefore, do not take the initiative to terminate your contract by means of a registered letter. If you do, your phone number may be cancelled without the possibility to transfer it.
Make sure to be in Belgium at the time of the number transfer. The transfer may go wrong or delays may occur if you are not connected to the Belgian network.
It is recommended to use most of your calling credit on your current SIM card before starting the number transfer. Calling credit cannot be transferred from one provider to the other.