ah, that makes sense....., i think..... maybe....
4G/VoLTE requires a few preconditions, one of them is for the telco to support it, even though the phone is VoLTE compatible. i guess, some kinda "profile", analogous to how iphone 5G is/was not supported on some telcos.
if Simba does not support it, then GG liao. you can check the simba website for the list of phones supported.
(it is possible that the phone may be supported although not on the list - see below)
Same for m1, which like Simba, uses 4G/VoLTE calling only.
ST (Zero1 infra provider) and SH have not turned off 3G yet, so it is possible that even though VoLTE for that phone is not supported, calls get thru because it falls back to 3G. The real test will come in late 2024/early 2025 when ST and SH have committed to start turning off 3G - at that point, if 3G fall back is needed, then calls will fail.
one way to test, is to turn off 4G (disable "VoLTE call") or select up to 3G connection (settings - SIM - "preferred network" - "2G/3G (automatic)". if the call goes thru, then it is using 3G fallback.
now that she is on Simba (I think they dun have 3G fallback), that could explain why calls dun go thru.
while m1/circles.life is on VoLTE only, it is possible that the phone is supported for calls. (i didn't check)
going forward now, if stay on simba, will need to change phone, or with existing phone, use ip calling like WhatsApp/Telegram/other app.
if go back to ST or SH infra, check with the telco whether your existing phone is supported on VoLTE. if not, then new phone is also needed. if so, then consider whether Simba is a good/value provider. if yes, then stay and get a new phone.
alternatively, if m1 supports the phone, then can consider moving to m1 infra telcos (m1, maxx, circles.life etc), and take a chance that they won't remove support in the future.
(note: when i compared their 2024 vs 2023 VoLTE supported handsets, a lot of older phones were removed. i am not on Simba, so i can't tell whether the unlisting of support really means the previously supported phone no longer works or not).
edit: or use 2 telcos, one SIM for calls, other SIM for data, but this increases running costs.
End of day, its a balance between new handset cost, running costs, and coverage (at where your mother stays and goes to).
long reply, hope it helps