Noted!
Every rule has its exceptions. 
Given Straight Talk and other TracFone brands use Verizon’s network, I figured once carrier unlocked it would work well on other Verizon network services.
I’ve since also tried an active Tello (T-Mobile network) SIM. As expected and as is often the case even with factory unlocked Android phones, visual voicemail in the default Google Phone app doesn’t work but everything else including WiFi calling seems to do so. I don’t currently have an active AT&T network SIM with which to test but may take a one month flier on these folks.