[Samsung Galaxy S20 / S20+ / S20 Ultra Questions & ] sjevtic:
I have a similar situation: I'm trying to get VZW Wi-Fi calling to work on an unlocked international S20+ (SM-G986B). Verizon told me they allow WI-Fi calling on BYOD equipment and that my account is properly provisioned. However:

There is no evidence of any Wi-Fi calling settings anywhere in the phone's UI.

System logs (e.g., logcat | grep -e UNWFC -e EPDG) show lots of info about not being registered for Wi-Fi calling.

IMS Settings show that VoLTE is provisioned, but not HD Voice. VoLTE clearly works.

Samsung's Wi-Fi calling stack is supposed to be pretty universal these days, and to a large extent, it delivers: when I put in a T-Mobile SIM, Wi-Fi calling comes up right away. So it seems that the Wi-Fi calling stack on my phone isn't completely out to lunch. But Verizon's Wi-Fi calling implementation appears to not be so cut and dry.

Last night I popped open a SM-G986U (supposedly universal carrier-unlocked US Snapdragon S20+) firmware image in an attempt to understand what Verizon is doing for their Wi-Fi calling, and it appears I opened a can of worms.. I found a whole lot of Verizon-specific content in there that I don't have.

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Just based on their names, a quick glance at the packages baked into system.img suggests the following are reasonably likely to be associated with Wi-Fi calling:

VZWAPNLib_VZW.apk

VZWAPNService_sku.ap

VzwImsApiService.apk

AdvancedCalling.apk

The big question is AdvancedCalling.apk?! Is this some sort of proprietary implementation of Wi-Fi calling and VoLTE?! Is it possible that Wi-Fi calling on Verizon isn't possible using the Samsung Unified Wi-Fi Calling app? And is there special APN config that needs to get pushed to my phone to enable EPDG connectivity required for Wi-Fi calling?

There's a whole lot in the CSC images (optics.img and prism.img) too. Of particular note, prism.img's imsupdate.json seems to define some sort of E911 IMS configuration. I wouldn't be suprised if that was a prerequiste for WI-Fi calling.

I also find interesting the declaration of packages allowed during power saving and emergency modes (suggesting these packages are necessary for core functionality like calling):

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Interestingly, I see references to the packages I called out above. The also are likely involved too:

com.ipsec.service

com.sec.vowifispg

Even the Samsung Unified WiFI calling app made the list.

Ideally, we need some input here from someone that has VZW Wi-Fi calling up and running on a rooted device (and who is willing to do a bit of experimentation!) that can help us understand what components are involved/required. I don't know if there is anything to block you from installing the universal or VZW-specific firmware on your device, though based on what I remember from past reading on the topic it seems likely to be a large step in the right direction; please confirm for yourslef. I'm in a bit tougher spot than you because flashing the VZW firmware simply isn't an option on my Exynos device. I also don't want any unneeded extra VZW bloat on my system, but based on what I've seen it looks like I'll have to merge at least part of the VZW CSC into my ROM image.

Speaking of rooting and custom ROMs, have you managed to get the bootloader unlocked? As I understand, this is a nontrivial effort for US-based Snapdragon models and currently only available as a commercial unlocking service. The feedback seems pretty good but it appears that the service is limited to a handful of models running particular bootloader revisions.