They work because they've been baked into the phone from the get go.
Bake it in correctly then. Nexus phones have worked with T-Mobile's wifi calling going back to the Nexus 5 if memory serves me right. And the 5X and 6P, which aren't carried by then, worked out of the box with it.If that was all it took, WFC for a TMO customer would work on any Android phone that has WFC baked in--Verizon variants of cross-compatible Android phones, for example. But evidently that's not the case.
Bake it in correctly then.
It's hard to comment on what stops it though. Is it certification? Or is it Verizon simply not wanting wifi calling to work on their variant of the S7 edge except for their network and Samsung obliges? Wifi calling worked on my unlocked Moto X Pure Edition and works on my Pixel XL.The problem arises when you have an OEM majorly involved in device software development/updates, and carrier-specific variants. Samsung, for example, may not bother with certification for TMO on a phone TMO isn't going to carry, including other carriers' variants of a Samsung phone TMO does carry.
The Nexus examples you gave were carrier-ambivalent, at least in that there was one model for all North American carriers. Same with the Pixel line and many other factory unlocked phones.