Google is disabling some Phone app features on older Pixel devices after acknowledging a bug that inadvertently leaked audio to callers. The issue has specifically been linked to the Take a Message feature released last year, which automatically answers and transcribes voicemails when you miss a call, with a handful of Pixel 4 and 5 owners reporting that their microphones were activating when callers were leaving messages.
Take a Message functions a lot like your traditional voicemail. When you either miss or decline a call, the caller will hear a message that says, "The person you have called is not available. Please leave a message." When they leave a message, you'll see a transcript in your call history and a play button to hear the actual audio if the transcription misses something (which it did when I tried the feature for myself).