Wi-Fi calling is messy for Singapore telecom providers.
Reference:
https://forums.hardwarezone.com.sg/...ly-plans-in-singapore.7167328/#post-157730027
Basically SIMBA seems to be the only one which may be more consistent in this aspect based on the reports.
1) ST MVNOs: no support of VoWiFi at all, even in Singapore
2) ST: only in Singapore
3) M1 and M1 Maxx: only in Singapore
4) StarHub: only in Singapore
5) SIMBA: works when roaming
6) eight: may or may not work when roaming.
7) dual SIM phones may have some issues on one of the SIMs even if both SIMs support VoWiFi.
And for eight and SIMBA, even when VoWiFi may work during roaming, no confirmation that outgoing SMS is possible or not. Hopefully someone can confirm.