Area codes or prefixes 800, 833, 844, 855, 866, 877 and 888 are used by toll free numbers in the United States, Canada and the rest of the countries using the North American Numbering Plan. A toll free number is a telephone number you can call free of charge (its owner, often a business, pays the calling charges).
Calls dialed from cell phones do not qualify for toll free calling - this may no longer be an issue since many wireless plans these days include unlimited long distance calls.
Vanity numbers are easy to remember toll free numbers such as 1-800-FLOWERS (1-800-3569377). These are generally used by businesses to achieve a better response rate from potential customers.
Yes, toll free numbers can be transferred from one telephone company to another.
For one to be able to dial a toll free number from a payphone without paying any charges, the owner of the toll free number is charged an extra payphone fee. This is why some companies block their toll free number from being accessed via pay phones. Often you can call a TF number via a payphone if you pay the payphone charges yourself.
Let's say you are in Canada and you want to call a toll free number registered in the US. Will you be able to dial? This is entirely up to the company/person who owns the US toll free number. They can opt to block calls from other NANP countries.
Again, depending on the settings used by the toll free number's owner, the number may or may not be accessible from other countries. If it is, the call is not (toll) free.
Some VoIP service providers such as Skype allow dialing US toll free numbers free of charge regardless of the caller's location. In other words, you may be in Europe and still be able to call without paying.