When someone calls your phone number or extension, your phone will ring with your normal ringtone. On Android, the client will take over the screen with an Accept or Decline button. On iOS, the new CallKit integration allows the standard Apple look-and-feel with the unlock slider to answer the call.
If you get an incoming video call, you will have buttons for Accept Video, Accept Audio and decline.
If you don’t want to enable video for the call, use Accept Audio. You can always add video to an existing call if both sides support video. Note the Add Video button below the Hold button below.