Did it work straight away? I just ported over not getting VoLTE yet.
What phone you are using?
giga VoLTE need phone firmware support.
For most recent Xiaomi phone that are base on Qualcomm chipset, the phone should have preloaded the StarHub VoLTE profile to the modem firmware.
My Redmi Note 4 SD625 using stock modem firmware do not support VoLTE on Singtel, StarHub and M1.
After modding the modem.img for Redmi Note 4 SD625, the phone is able to load the StarHub VoLTE profile at modem firmware level and the VoLTE works thereafter.
For other phone like iPhone and most recent Samsung, StarHub previously may have disabled the VoLTE profile at modem firmware level.
If the VoLTE doesn't work for you, means you need to wait for firmware update from your phone manufacturer to get VoLTE support.