At a business, a secretary, almost always a woman, would answer the phone for all calls. There were no direct lines into anyone. If you were in she would transfer the call to your office. If you were out she would use this pad to take messages. When you came back in to the office you would always stop at the front desk and you would be handed your messages.
They’re still used in a lot of places from what I can tell. Healthcare settings for example, where the reception staff might not know what info is or isn’t sensitive.
Write down the message, note the patients file number, and then give it to the recipient when they are available. They can call back to discuss the details.
Sales jobs too, having a receptionist take the message and not having to leave voicemails makes the whole operation feel more personal.