Well, here is my point of view: For everyone who is complaining ASUS can't run VoLTE, if you go check your phone, there are showing 4G on your mobile voice service type. BTW I'm using Zenfone 5z.
So I'm guessing the local telco didn't consider to open VoLTE for ASUS because they are small market phone. Brand like this may struggling to let those telco open VoLTE to them because they have smaller market. Same goes to Sony (refer to Maxis), only the newer model of Sony is supported. Ps. Well, let me know if I'm posted something wrong information.👌