| Area Code Information Detailed information on the history and current developments in North American NPA allocations. http://www.areacode-info.com/ |
| International Numbering Plans A collection of all international numbering plans in the world. Some features require a paid subscription. http://www.numberingplans.com/ |
| 800 Service Management System Information about the central database for US toll free number management. http://www.sms800.com/ |
| The Telephone Exchange Name Project The historical relationship between North American telephone exchange names and numbers. http://ourwebhome.com/TENP/TENproject.html |
