“WiFi calling” is a term that’s used specifically to refer to a kind of VOIP that is provided by your cellular network provider (carrier). By definition, it connects to servers provided by the cellular network provider and authenticates using the SIM card in the phone.
Its primary benefit is seamlessly improved coverage for the conventional cellular service into all locations where the device has access to the Internet using WiFi, using the same telephone number and the same billing account, with no setup or configuration, and no need to load a separate application.
This contrasts with other VOIP services, which tend to have advantages such as low cost, privacy and flexibility, but cannot normally make direct use of a telephone number provided by your cellular network provider, and tend to operate as a separate service.
Of course, you can achieve functionality similar to WiFi calling by using a standalone VOIP service as your primary telephone service, but it requires a different arrangement of service providers. My question is specifically about WiFi calling.
I make no claim about which solution is best. I think both have valid use cases.