No app wil ever accomplish consistent good voice quality over the internet. Ever. For that it's too much of an unreliable network. You may try some calls over Signal, think Hey! This works well! until you hit a moment where the route between endpoints is flaky, attribute it to Signal while using another app for that same call at that particular point in time would have given the same result.

Only when QoS will be honored by all internet routers will we reach something that is consistently reliable.