Toll free numbers are used within a country's national borders to facilitate toll free calling. They are not technologically designed to be dialed free of charge from outside the country where they are registered. It's the reason why some toll free number owners choose to block receiving international calls and often advise foreign callers to contact them on a regular telephone number. Moreover, some local carriers will not even route calls to a foreign toll free number.
The above being said, you can try dialing a foreign toll free number and the call will sometimes work. The same dialing sequence used for calling internationally applies: exit code - country code - toll free number . Be sure to leave out any initial zero in the toll free number.
If the call goes through you will incur regular (or sometimes higher) international charges. So, if the company/person you want to reach also has a regular telephone number it is better to dial the latter when calling from another country.
Normally, to call a toll free number registered in the US, Canada or any other country using the North American Numbering Plan (NANP) from abroad you should use the dialing instructions we provide on this site. If the call goes through you are usually informed that your call isn't free.
Another way to dial North American toll free numbers from abroad is by replacing the 3 digit toll free codes with other codes introduced years ago:
These codes were originally introduced for calls to toll free numbers dialed between NANP countries, allowing the caller and the call receiver to each share their part of the cost (the caller would pay international charges and the callee would pay their national toll free rate). Moreover, according to this official announcement, the codes were supposedly retired in 2004. However, I've been getting reports stating that, not only they still work as described above, but it seems that they can be used to replace regular North American toll free codes when dialing from any country, and not just countries using the North American Numbering Plan.
International Freephone Numbers are an exception to the rules above. These numbers are assigned a separate country code which is 800. Calls to these numbers are free when dialed from certain countries that have adopted this system. Not many companies have an International Freephone number due to high costs involved with subscribing to this service.
Dialing an international toll free number works the same as placing an international call to country code 800. For example, dialing from the US you would enter: 011 800 ???? ????.
Read more on UIFNs.