From what I remember (and this was a while back), wifi calling establishes an IPsec tunnel to the cellular provider (using your SIM for auth) and tunnels GSM f

From what I remember (and this was a while back), wifi calling establishes an IPsec tunnel to the cellular provider (using your SIM for auth) and tunnels GSM frames over it. It's intended to let you handoff between "real" towers and wifi networks the same way that you move between one tower and another, which I assume means that it has to implement some functionality that is normally hidden in the baseband processor.