for my area.  I scan the area code & the location.  If I'm not sure I look at the ad link & often that gives a more detailed area.  There are very many area codes common to my region & cells can have any.   Also scan your local regional board.  
You are correct, it's one of the challenges of the research.
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Not THAT kind of provider. TELEPHONE service provider: http://www.fcc.gov/cib/consumerfacts/numbport.html

With number portability, area code is almost useless to find someone's primary location anymore.

You should check your local Regional Board for current listings, weekly ads, and discussions. (What's the local board for Knoxville?)