I've created an Address Book plug-in that enables you to get some details about the phone number selected, if the number has the international prefix included. Basically, it needs the format +n[nn] xxxxxxxx, where n's are a valid international prefix. The popup will show something like: 'Get details for number in "; selecting the action will take you to a detailed page for that number on numberingplans.com.
You can download the script from the macosxhints' file collection, and then open it in Script Editor. Once it's opened in Script Editor, save as Script in ~/Library/Address Book Plug-Ins, and it's ready to go.
Enjoy!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040216202919929