If I'm understanding how 'WiFi Calling" works; it's still "identifying you" to the cell provider the same way; via your SIM. The only difference is they don't get an exact location because you're not using any cell towers typically.
I do suspect SIMs and eSIMs are still doing all the heavy cryptographic signing done on a typical phone network though...they're just not screaming your IMEI/IMSI all over open or even encrypted airwaves; nor is a WiFI signal triangulate-able typically due to it's short range.
Actually; (basically) SIP over (basically) IPSec sounds pretty correct. Wish the dense technical manuals I read had explained it that way; makes a lot more sense to me as a Net Admin type of IT person.
I do remember reading that the protocol was basically encapsulated. Dunno about any encryption; probably there's not any at the IPSec level. I do know that the SIMs themselves probably contain certs that have some value; I just don't know if they handle any encryption or if they're just lightweight little numbers for authentication only.