Black mailed by my BFF over the phone.

So recently my so called "best friend" (let's call her Amelia) rang me and began to ask me all these questions about other people and so as it was just us two on the phone (so I thought) I responded honestly. I have now found out that there were 7 other people listening on the conversation. All these people heard what I had to say about them privately. I only know this because one of the girls listening told me because she said she felt guilty.