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thianhockNo info needed, I'll take your word for it, unfortunately with ASUS it's a lottery with each new smartphone and new firmware, there is no VolTE for the majority of operators and when it worked on a ROG3 it didn't is no longer the case on a 5 or a 6
Have you tried sending this SMS to Umobile and if it still doesn't work, wait until Monday and tag a moderator
If this happens, please SMS < WIFICALLING ON > to 28118 to activate the service.