Press the right hand side of the silver square selector button to bring up the menu options.

Using the square button, scroll over to the services icon and then press the button below ok.

Using the square button again. Scroll down to Call Waiting and press ok.

Using the square button again set call waiting to off and then click send.

The following screen will appear. Leave this for a few seconds and then press the Red phone icon to exit. Call waiting is now disabled.