I’m not an expert, but for example: “All calls on WhatsApp are VoIP”, and this means to me that you need an App (on both sides) to establish telephone call with someone. Just in comparison, VoLTE doesn’t need an App for telephone calls and link to one of explanations is here: http://www.techadvisory.org/2020/06/volte-what-is-it-and-how-different-is-it-from-voip/.