I've never heard of these codes being used in actual practice, but the following info might be worth a shot.
"Toll-free services are essentially automated collect calls. The recipient of the call pays the cost. However, many toll-free numbers only work within a limited subset of the North American Numbering Plan countries (for example, only the U.S.A., or only the U.S. and Canada), so "replace" codes have been implemented for the first three toll-free codes: 800, 888, and 877. To dial a U.S.-only 877 number from another NANP country, a caller might dial 1-882-nxx-xxxx. The caller pays the same international call charge as for a standard call to the United States. These codes may also be used for calls originating outside the NANP. However, the assignment of further 88X codes corresponding to additional toll-free codes is not guaranteed. In particular, 866 was activated 2000-07-29, but no replace code was issued (883 being the obvious candidate). Furthermore, the three existing 88X replace codes may be recaptured at some future date. If additional toll-free numbers are needed, 855, 844, 833, and 822 will be activated in that order, but replace codes will not be issued."