Hi Shir,
These Access Codes (0 or1) are really set up as part of an "overall" Standards based Numbering Plan, not by the Long Distance provider. So to complete any "Offnet" call these Standards have to be met before the call can be routed through the PSTN.
Although the International Telecommunication Union (ITU) has attempted to promote common standards among nation states, numbering plans take different formats in different parts of the world. For example, the ITU recommends that member states adopt 00 as their international access code. However, as these recommendations are not binding on member states, some have not, such as the United States, Canada, and other countries and territories participating in the North American Numbering Plan. Another significant country that has not changed is Australia.
The structure is:
Access code (either international or national) - only necessary when dialing international and "national" (non-local domestic) calls. The national access code is often quoted as if it were part of the telephone number. The most common national access code is "0", and the most common international access code "00"; in the North American Numbering Plan (e.g. the United States and Canada), however, "1" and "011" are used respectively.
Country code - only necessary when dialing to phones in other countries. It is often quoted together with the international access code which must precede it in the dial string, especially in the United States and Canada (e.g. "011-XX-YYY-ZZZ-ZZZZ"). In international usage, telephone numbers are typically quoted with the country code preceded by a "+", and spaces in place of hyphens (e.g. "+XX YYY ZZZ ZZZZ"). (On GSM networks, "+" is an actual character that may be used internally as the international access code, rather than simply being a convention.)
Area code - only necessary (for the most part) dialed from outside the code area, from mobile phones, and (especially within North America) from within overlay plans. Area codes usually indicate geographical areas within one country that are covered by perhaps hundreds of telephone exchanges. It must usually be preceded in the dial string by either the national access code or the international access code and country code. Non-geographical numbers, as well as mobile telephones outside of the United States and Canada, do not strictly speaking have an area code even though they are usually written as if they do.
Local number - must always be dialed in its entirety. The first few digits in the local number typically indicate smaller geographical areas or individual telephone exchanges. In mobile networks they may indicate a network provider in case the area code doesn't. Callers from a number with a given area/country code usually do not need to include this particular area/country code in the number dialed, which enables shorter 'dial strings' to be used. This is an issue when the number must be keyed by hand, but where the dialing is automated (increasingly common) it is not an issue - and it is arguably better to include the full number with access codes in devices that dial automatically.
From this excellent Wikipedia doc;
http://en.wikipedia.org/wiki/Area_code
Hope this helps!
Rob
