Just ask Braxman.
BTW, VoLTE-capability doesn’t depend on the OS, per se.
Be aware that a phone may get VoLTE on one network, but not another. For my Android, the VoLTE setting is present in the menu when I have a T-Mobile SIM inserted (or a T-Mobile-based MVNO), but not when I have an AT&T (or MVNO) SIM inserted. That’s either because the OEM vetted for T-Mobile’s network, but not for AT&T’s, or because AT&T would demand to whitelist my device first… I don’t know which.
The device’s regional firmware can also affect the presence of VoLTE; if you buy a phone produced for Europe, it may not have the same modem (EDIT: or firmware), and it may not be approved for VoLTE on a U.S. network. And vice versa. (In some cases, you can replace the firmware with a different version, which I imagine Braxman can do, too.)