There are 2 possible numbers: +32486000005 or +32456002000. If one doesn't work, just try the other.
iPhone users can set the SMS centre by entering the code *5005*7672*+32486000005# or *5005*7672*+32456002000# and pressing the call button. Hold the 0 for a few seconds to make the + sign. Doesn't work? Add an extra * at the start of the code!
For all other operating systems, users can enter the number of the SMS centre in the settings of their device (usually in the settings of your text app).