Answer:
Toll free telephone numbers begin as follows: 800, 888, 877, and 866. The related link below provides an explanation of what toll free numbers are and how they work.
Toll free numbers are numbers that you can call for free of cost. You can avail some of the services without spending money. For example, if you want any Indian railway enquiry, you can call 131. Like wise, most of the organizations providing toll free numbers.
Most business use toll free number to support their customer service. The owner of the number is the one who will be paying for the minutes used during the call,instead of the calling party.
In some regions such as UK, they use 0800 as prefix. China also use 400 toll free prefix aside from the universal 800 number.
Although 800 is the most popular prefix for a toll free number, there are also other prefixes that are being used now, which are 888, 866 and 877. There are no differences among these numbers in terms of features. However, an 800 number is seen to be better than the others. Toll free numbers with the prefix 800 are seen to be more effective and powerful when used in advertising and promotion, making them more expensive as well. A more extensive discussion on the differences among toll free number prefixes can be found here.