If I don't recognize a telephone number through my caller ID/contact list, then let it roll over to voice mail. I let people know in my voice mail announcement that "I'm either busy or I do not recognize your phone number." Then I ask that they "please leave a message" after which I will check my messages.
If it's someone I know, I'll return their call and set them up in my contact list.
This individual did not leave a message.
United States