I had the same problem today.
I returned to Straight Talk service after a few months using T-Mobile and Google Project Fi service.
My phone is Motorola Nexus 6, a pure Google Android 6.0 phone which has WiFi calling function.
I tried to enroll my phone to get Straight Talk wifi calling service, but just as other reported, it said my phone was not good for it.
I had wifi calling service with the same phone when I was using T-Mobile service.
I chatted with Straight Talk customer service tech support,
and got the answer was: Straight Talk does not support wifi calling service to the Bring Your Own Phone customer.
Is it fair?
We pay the same price but get less service!
I think I will go back to other carrier to get wifi calling even VoLTE service.