The North American Numbering Plan was created in 1947 to simplify long-distance calling and make it something that people could do without needing to go through an operator. Used in the United States, Canada, and numerous Caribbean countries, the plan was built around the addition of a 3-digit NPA code—commonly referred to as an area code—preceding 7-digit local numbers. Other than a few special exceptions, NPA codes are assigned based on geography.
Toronto's Area Codes
The City of Toronto is currently served by three area codes:
416: This was the original area code assigned to Southern Ontario by the NANP in 1947. Over the years, other parts of Southern Ontario were given their own area codes and when the 905 area code was created in 1993 for the suburbs around Toronto, only the Metro Toronto area—which is now the amalgamated City of Toronto—was left with 416 numbers.
647: This second Toronto area code was added in March 2001 as an "overlay" to 416, serving the same area. With the addition of this area code, anyone calling locally in Toronto had to dial all ten numbers.
437: Toronto's third area code was added in March 2013. It is another overlay number, and there is no geographic distinction between it and the 416 or 647 areas. All three numbers can be assigned within any part of the City of Toronto.
GTA Area Codes
The Greater Toronto Area is covered by three overlaid area codes just like Toronto, meaning GTA residents also need to dial all ten digits, even for local calls.
905: In 1993 the 905 area code was split from the 416 and phone numbers for those in the suburbs surrounding Metro Toronto were changed to begin with 905.
289: In June 2001, the 289 area code was created as an overlay with the 905. After this, those living in the Greater Toronto Area were also required to dial all ten numbers, even when making a local call.
365: In March 2013 the new area code 365 was added as an overlay to 905/289. When new overlay numbers are added, they are not actually assigned to customers until the old numbers run out.
When the Area Code Doesn't Matter
Although you usually need to dial all ten digits to make a call in Toronto or the GTA, you don't need to worry about area codes when calling one of the special 3-digit phone numbers such as 911 (in an emergency), or 311, Toronto's Municipal Information and Services hotline.
Find More Area Codes
If you're looking for information on area codes in other areas, About.com: Web Search has tips on How to Find an Area Code Using the Web.