Did you answer the call?  What was it about?  Do they leave voicemail when unanswered?

Have you tried turning on the CallControl  feature?

https://www.koodomobile.com/en/help/call-control

if the “unknown caller” was from an actual person, they can enter the code requested from “call control”, 
Dedpending which phone you are using, you could silent “unknown caller” by going to settings » phone » silent unknown caller if you are using an iPhone. If you are using an Android , going the phone dialer app » select the tri-dot on the top right of the phone screen » select settings » block number » block call from unknown numbers.