The voice call quality can be quite different. The muffled voice of yesterdecade is much less likely when there's VoLTE on both sides. No, not even HD Voice required, but of course it's even clearer with HD / FHD Voice support.
Sometimes I gave up repeating what I was saying when calling via SingTel 3G, I told the other party to give me a min, I call again via M1 / SIMBA 4G (VoLTE). No, it's not the device's issue, because I'm calling from the same device, just using the another SIM slot to call.