International Numbering Plans
- A collection of all international numbering plans in the world. Some features require a paid subscription.
- http://www.numberingplans.com/

Area Code Information
- Detailed information on the history and current developments in North American NPA allocations.
- http://www.areacode-info.com/

Number Portability
- Collates information on the regional initiatives in place to develop number portability and number pooling in the US.
- http://www.ported.com/

The Telephone Exchange Name Project
- The historical relationship between North American telephone exchange names and numbers.
- http://ourwebhome.com/TENP/TENproject.html

800 Service Management System
- Information about the central database for US toll free number management.
- http://www.sms800.com/