Basically, phones that support LTE or 4G are capable of VoLTE.

Perhaps it was disabled on devices since that was before VoLTE protocol was established. As you know the LTE is just data streaming like packet network in Internet. VoWiFi is Voice over WiFi which is something like you make call through WiFi data network. I think like Viber, WhatsApp, etc. The LTE is data through phone network. The 4G is the starting point. The 3G is still phone switching network to transport data.

See if this help in your case

Developers forum usually have the apps usually. I could not recall but on some devices there is apps to enable this.

Easiest is to get enabled phone