that is very strange, esp on a paid plan. sounds like backend problem.
going from the anecdata here, eight may have backend/provisioning issues. like cannot get otp from eight after porting in, receiving overseas sms etc.
best to escalate - fastest reported escalation seems to be at ubi.
Thinking a bit during tea break, it may not necessarily be a backend problem.
@wulu1_69 what phone are u using?
when roaming, your phone needs to be on the roaming partner's supported VoLTE handset list.
I assume you have no problems with VoLTE enabled in sg (when on eight), and that it is only in JB.
What you can do is to check Maxis or U Mobile, what are their supported handsets.
if your handset is not suported but has VoLTE capability built-in, then you probably won't be able to call out/receive calls, but you
may be able to send/receive SMS. The telco will usually tell you to "upgrade" your handset.
I know, technology is supposed to make our life easier, but small things like this are a pain.