Contacting the owner of a FSBO when the number on the sign doesn't work

[Phillip Syrios]

I saw a duplex that had a for sale by owner sign in the front yard that fits perfectly into the type of property we like to buy. The problem is when I try to call the number on the sign I get a message saying the number is "unreachable."

I have looked up the owners on the local parcel map and am sending them a letter which in my opinion is a long shot to make contact with them, any better ideas?

[Aaron Montague]

@Phillip Syrios 

Knock on the door.  
Leave a business card.  
Ask the current tenants where they send their checks.  
Talk to the neighbors.