I was looking at the permissions that many system apps and some user apps have access to mic and phone. I don't remember granting those. I think that some apps have WRITE_SECURE_SETTINGS permission and are misusing it.
My device is Vivo V2029 running Android S. I know that now this permission cannot be granted to a non system app without adb or root. My device used to run older android version before, maybe it was granted then.
I was thinking of using adb to revoke it for each app one by one and check if the device functions properly. But I am concerned that what if I revoke it for some important app and then even after re-granting, the phone becomes unusable.