@kaan Element (Matrix client) does video/audio calling. A 1-to-1 call will be point to point, so quality will depend on how the phone handles WebRTC, which it's built on. Also if your call routing has to fall back to using a TURN service (eg because you're behind CG-NAT) then quality will drop. These same problems likely affected your signal experience. A call between IPv6 enabled endpoints with good bandwidth yields best results.
@kaan A group call of 3 or more will require routing through a video bridge, where again like the TURN service, quality will depend on an external party.
Point is it's not simple, point to point VoIP is made difficult by all the shitty ISPs and carriers that have languished on IPv6 rollout and instead opted for lazy band aids like CG-NAT.
WhatsApp and co are easy/look good because they poor money into video bridge backends to placate and entrap their herds of product.
Balsillie family social hub.