I spotted some difference with older and newer giga subscription related to VoLTE support.
For my older active giga subscription, I'm able to connect to the proper "ims" APN and I'm issued with an Internal IP that cannot be use to surf net. This means that my older giga subscription can connect to the IMS network APN and this is why VoLTE works.
While for my newer giga subscription, the "ims" APN connection issue me a Internet accessible IP which I can use to surf net. This is why my new giga subscription cannot connect to VoLTE.
Maybe to check if you are able to connect to VoLTE network, you an create a new APN call "ims" and the APN type is "default,supl".
If the 4G Data Logo show up and you can still surf net, means your subscription do not have access to VoLTE network.
If the 4G Data Logo show up and you cannot access to Internet, means giga has enable VoLTE access to your subscription. Just that the phone can't support the VoLTE at the mobile phone firmware level.