You can turn off your own caller identification, your number will then not be shown at the receiving end when you make a call. Your number is only hidden when you make voice calls not when you send messages.
Press the phone icon.
Press the menu icon.
Press Supplementary services.
Press Show your caller ID below the required SIM.
Press Always to turn on caller identification.
Press Never to turn off caller identification.
Press the Home key to return to the home screen.