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Hoping that ASUS decide to include these settings to support Australian carriers in the next firmware update (when ever that is??) .
If I am reading correctly they certainly have included support for Australian and other carriers in the ROG 5 firmware according to the Qualcomm modem mbn config files.
As other users have stated this is a very basic feature in budget phones and I would have thought been there in a flagship phone. Telstra here in Australia certainly don't have a white list per say as to who can and can't have VoLTE access and VoLTE access is automatically enabled on Telstra accounts there end.