OnePlus 6 update optimizes call quality, fixes various issues
If you've already received a new update tagged OxygenOS 5.1.8, then you should expect better call quality. According to OnePlus, the update is meant to optimize this feature, as well as fix some stability issues affecting the OnePlus 6.
Also, OxygenOS 5.1.8 update should improve network stability and adds some general bug fixes. Finally, OnePlus mentions that this specific build includes all the features that have been added in the OxygenOS 5.1.7 update (for India region).
Keep in mind that since the update is pushed out in waves, it might take up to a few days to reach all OnePlus 6 devices, so it's not available yet on your phone, give it one or two more days.