starhub has finally started to phase out 3G.
edit: since you can use data, send/receive sms, only cannot make/receive calls, this points to a 4G phone where the telco does not support your phone model.
yes, the issue is at starhub backend. for volte to work, in addition to the phone having the technology, the telco must also support it. (assuming the bands on the phone are also supported by the telco. if the phone is IDA approved, then it definitely will).
since singtel supports it, then the phone is capable of it.
looking at the starhub supported model list (not supported does not mean the phone does not have VoLTE capability), it states samsung year 2018 launch models and earlier are not supported. The A30s (2019 launch) may fall on the border.
(for the list:
https://www.starhub.com/personal/support/article.html?id=nWfxXtEMRM7Xj1iJGKvtl8, scroll down to the section "
3. What is VoLTE?" and expand the section)
elsewhere, the site offers to sell you a phone that they support. the message is obvious.
edit: for number portability, it means you need to buy a phone which is suported by the telcos you intend to port to in the future.
you also need to anticipate whether the telco will withdraw support of the phone in future (if your telco does, then you need to either get a new phone or port out)
additionally, you also need to consider whether the phone is suported by overseas telcos if you intend to travel.
this effectively limits number portability and the unrestricted mobile market where you are supposed to be able to use any appoved phone at any telco. If a telco only limits support to models they sell, it effectively creates "telco locked" phones, something which then IDA prohibited.
overseas, the telco can "whitelist" the "unsupported" phones IMEI, but overhere, i doubt there is an incentive to do so. it means that many perfectly useable phones will be tossed, and an unecessary new phone purchase will be forced on users.
IMDA may not even be aware of the situation which they have created.
good luck.