How secure is a cell call when using WiFi calling?

I've been looking for information on this topic for a while but haven't found anything substantial. If a user makes a call on their cell phone when using WiFi calling, that voice traffic obviously moves over the network for some amount of time. During that time, how vulnerable is that data? I've seen it stated that the cellular provider encrypts this data, but I have not found any information regarding the mechanism, cyphers, protocols, etc. that are being used. Can …

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