I have Note10+ and S21. Also using iPhone.
All no issues.
Also u need to check how old is the information about Xperia 5iii not able to use VoLTE in Maxis as I saw their list keep updating. So at the time of post was written maybe true but if after the list was updated after then the information will not be updated. You can try to msg the person who post it to ask about it.
If not mistaken, Xperia 1ii in their list not updated initially (saw some complaint about it in this thread) ... then months after saw it listed.
Suggest to go with all the prominent Android smartphone brands like Samsung, Huawei, Oppo, Vivo, Realme and Xiaomi ... and I assume those models that sold on Maxis website should have VoLTE, guess they won't be so screwed up to sell non- Maxis VoLTE smartphone in their own store.
Also point to note Maxis recently launched VoWiFi and the list also on their website ... and the list is way shorter than their VoLTE one ... so if you want both features .. you should aim for those appear in both of their list ...
Thanks for the response. And those who responded as well. Appreciate it. Will check the VoWiFi device list.