It’s highly likely there’s something wrong with your SMS centre, so you need to check.
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.
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).
Afterwards, reboot your device and try sending an SMS to yourself. If that doesn’t work, do let us know.