It is the two-way walkie talkie (Direct connect) option. You have to have another persons two-way/direct connect number, it is different than their phone number, its usually something like: 123*4568*9 Once you have their two-way number you just input it, press and hold the side button on your phone and after the beep, beep you then talk, when you are done speaking you let go of the button so they can respond. Not many people use direct connect anymore, it was all the rave about 8 years ago.
