The voice call is a total mess. If I'm not online, the person who's trying a WhatsApp voice call hears the call ringing while I don't really get a ring in my phone. This gives all the wrong impression to the person who is calling, that I'm not attending his or her call while I didn't really get a ring in the first place!. Try sending a voice message to the person calling that I'm not online currently in this case. Hope you fix it.