Unlike the UK’s Royal family, I doubt anyone is listening in on my phone calls. But, if someone wanted to, they could.
Many mobile phone calls in the U.S. and Europe are encrypted with a stream cipher called A5/1, which is commonly used, in GSM (Global System for Mobile Communications) voice communications. A5/1 is not secure. It’s been broken for years.
More recently, in 2009, A5/1 was busted by a German hacker in a way that demonstrated that if you can capture the voice stream anyone with generic computer equipment could break it. Since then, A5/1 has only gotten easier to crack.