Toll-free numbers in the North American Numbering Plan (NANP) are sometimes called "One-800 numbers" after the original area code which was used to dial them. They include the area codes 800 since 1967, 888 (since 1996), 877 (since 1998), and 866 (since 2000). Area codes reserved for future expansion include 855 , 844, 833, 822, 880 through 887, and 889.
免费电话号码包括800,888,877,866开头的电话。
英文摘自:http://en.wikipedia.org/wiki/Toll-free_telephone_number
补充:但并不是8开头的都是免费的,另外,号码有时前面加个1-,可能表示美国、加拿大的区号,也是免费的,比如1-800XXXXXXX。
9开头的电话号码除了电话费可能还有另外收费,被称为Premium-rate telephone numbers,这要看提供这个号码的是怎么规定的,比如,有的9开头的号码是Call Collect,即对方付费电话,过去需要接线员,在《成长的烦恼》上有类似的片断,现在可以自动完成,这项服务被称为International Freephone Service。