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Once you know their phone number you can find their location by area code (or use a Web service to get more info on that number).
Looks like a good article. But it seems to me the best way to prove location for most needs is by telephone number. Let the user provide a phone number, then call them on that number to verify that they are at the phone number they say. You can automate this with a readily available and inexpensive Web service. (Try a Web search for "telephone verification web service")
What if they're using a mobile phone? Also, some area codes (in the US and Canada) span large geographical areas. Further, it isn't always practical to take phone numbers; people change phone numbers; people use multiple phones.
Once you have verified the phone number you can learn all about the number and its owner; do a Web search for "intelligence single phone number web service".
How does it verify the phone number, especially if the number is mobile or otherwise portable? How quickly are changes in registration made to the database mapping phone numbers to individuals? (Note these are among the same problems cited by the geolocation paper. It seems that if the problems cited are easily surmountable with phone numbers, there would be a strong push within industry to use phone numbers in geolocation.)
As an aside, since search engines and ad networks are using geolocation (presumably) to determine what ads to place on results pages, it isn't practical for them to require users to submit phone numbers.