I encountered a strange ringback tone when calling my wife today. When I went to hang up I noticed a message on the call screen: “Call encrypted by Google Fi.” This is the first time I noticed this, and when researching I found in a help article:
“When you use an Android phone on Google Fi to and are on a call with another Android phone on Google Fi, the call stays private between the two of you with end-to-end encryption. The audio is encoded, and can only be decoded with a shared secret key. Each device has a private key and a public key, which aren’t saved.”
Both my wife and I are Google Fi users. Just thought it was interesting.