That's called the "Phone" setting in the menus. You can turn it off in Menu >> Settings >> Phone >> Status = Off. Also, if you want more direct access to that, you can add it to the circular Controls screen that comes up by long-pressing the Light button. It might already be there - it's the icon with a telephone handset and bluetooth icon. If it's not there, long-press the Up button then select Add Controls >> Phone.
