A team of academics from the Ruhr University in Bochum, Germany, has discovered that not all mobile operators follow the 4G standard to the letter of the law: they supports encrypted voice calls, but many calls are encrypted with the same encryption key.
Article Link: https://www.andreafortuna.org/2020/08/14/revolte-decrypting-lte-calls-to-eavesdrop-on-conversations/