Moto Z Play VoLTE option?

[Wyveryx]

I've been using a Moto Z Play with little to no issue for nearly a year, and prior to that I was using a Nexus 6. These have both been with the VZW sim since my area isn't well covered by other towers.
I had used the Nexus 6 on VZW itself and was able to do data and voice at the same time, but not so on Straight Talk. I always chalked it up to an APN setting or limited network ability. However, now that I switched to a Moto Z Play I am noticing more and more that I try and use this feature. I know the phone is capable and I've seen posts that verify that Straight Talk's network is capable, now I only need to know how to enable it.

Is it a specific APN setting, or is it specific sim only ability?