During qualcomm 625 generation, you are paid (for software driver) to unlock VoLTE feature, because during that period, China mainland just pushing VoLTE to forcing more customer using 4G instead of 3G because of phone call, as it is quite unstable to force enable it during that time yet, you may get no call in and fallback to 3G/2G.

For example, Mi Max 2, it required to rename the driver name to unlock VoLTE feature that works.
And nope, Xiaomi don't unlock it until stop maintaining it.

There is another phone like Realme C2, 450(downgrade frequency of 625), paid for unlocking VoLTE and VoWIFI for India Jio at last update, but locked it for Jio telco only.
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Anyways, the worst is not no VoLTE, but failed for fallback to 2G.
Try calling someone and found out you can't call him, only because he got a crappy/buggy VoLTE enabled, and no way to workaround it.

And yes, maybe WhatsApp that guy can be done, but still...

That's why got whitelist, but don't think it works as it should be, based on some person I failed to call after shut down of 3G...