You said no message was left.
How do you know the caller knows you are assigned your number?
There is nothing special about your telephone number, anyone can call it at any time.
What is special is that your name (or address) is associated with your telephone number.
Unlisted and unpublished numbers are an attempt to decouple the association.
Scammers and criminals, though, are always attempting to find out more information, and so they call all possible numbers. It is called vishing.
https://en.wikipedia.org/wiki/Voice_phishing