Here a bad analogy which should help explain:
VoLTE is like PTCL calls but on steroids (assuming both you and the receiving party are on a similar level). It's consistent and generally reliable. Does not require an internet connection, and can be used to call other landlines or phones.
WhatsApp calls and other VoIP calls are - well - VoIP. Latency matters, and each app has their own preferences as far as sound quality is concerned. Some sound softer but clear (e.g. WhatsApp - which, in my experience, has the most latency vs. other apps) some sound harsh but clear (e.g. Skype, Duo/Hangouts), some sound good even on poor connections (Teams) some don't sound well in poor connections (e.g. Zoom). But all are free, are bound by the network effect (no cross-network calls) and only require an internet connection. And in my experience, as I said, WhatsApp has a higher latency than the other apps (I had tested it myself a year or two back using an audio recorder and checking the wave samples between the sound sent from one phone and received at another, and recording the audio waveform via my computer's mic, and analyzing the delays - it may not seem much but it makes it a bit unnatural while the rest sound as natural as standard phone calls as far as delays are concerned - not sure why).
So - it's not about which is better, but which fulfills your requirement. They both can. Or not. Depends. 3G and VoLTE have very similar sounds (because of AMR Wideband) but 4G will be a ton more consistent than 3G (which falls down to 2G), as are other VoIP calls.