By default your number is displayed on the screen of the mobile phone of the person you are calling.
Permanently deactivate number display
You can hide the identification of your number in the settings of your phone.
If you have an iPhone, go to Settings, hit Phone and then Show My Caller ID. Toggle the option off to make your phone number private. If you change your mind, you can always turn it back on later.
Note: Your number will still be identified in case of calls to emergency services for legal reasons.
Deactivate number display per call
In case you do not want your own number to be displayed on the screen of the person you are about to call, send the code and the number you are going to dial in the following way:
(#)31(#)[phone number]