Kyocera Digno 3 (902KC) woes - USA VoLTE support?

I imported a sim-unlocked Kyocera Digno 3 (902KC) on a whim after seeing reports on here that the phone was compatible with US networks and that people were getting VoLTE. Unfortunately, it seems as if this is not the case for me - I've been testing it with a FreedomPop GSMT (basically a Red Pocket GSMT) sim and while SMS and data work perfectly on T-Mobile LTE, outbound and inbound calls fail unless I switch the phone to 3G/2G mode, which falls back to GPRS due to the 3G shutdown (yikes). I've read that the 903KC model may have better North American compatibility according to the T-Mobile IMEI checker, but considering specs and band support should be identical save for the software (903KC being Y!Mobile, 902KC being SoftBank), it's a bit strange to me how much trouble I'm having with getting VoLTE calls to work. I've ensured that 4G calling is enabled in the phone app, and I've tried sticking the SIM into some VoLTE compatible phones I have (iPhone SE1 and Jelly 2), placing some test calls, and then transferring it back to my Digno. Has anyone else with a 902KC or 903KC run into these issues? Is there some hidden setting I need to find and enable or am I SOL? Or if it's just a software issue, would it even be possible to flash 903KC software on the 902KC given I can even find a download link somewhere?

Edit: managed to get into the android network testing menu and IMS shows up as not provisioned along with VoLTE unavailable, yet voice network type shows up as provisioned and using LTE..