Sources of information on the history and development of worldwide public telephone numbering plans. 1. 800 Service Management System - Information about the central database for US toll free number management. 2. Area Code Information - Detailed information on the history and current developments in North American NPA allocations. 3. International Numbering Plans - A collection of all international numbering plans in the world. Some features require a paid subscription. 4. Number Portability - Collates information on the regional initiatives in place to develop number portability and number pooling in the US. 5. The Telephone Exchange Name Project - The historical relationship between North American telephone exchange names and numbers.

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