Remote control can be enabled in the app settings.  

To turn on remote control on your iPhone, go to: Menu>Settings>Push To Talk button>Headset/ remote control

The switch controls if Zello responds to remote control buttons. When it's turned on, if a user has a headset plugged into the audio connector that supports remote control, they can use it to control the talk screen. The play button can be to toggle the talk button and fast forward and reverse buttons can switch between recent contacts.