This information originated from nanog@merit.edu:

OUT OF DATE INFORMATION FOLLOWS

the following information is out-of-date. These numbers were withdrawn in NANPA Planning Letter 331[pdf].

For the record:                                      

  US     INTL
1-800 = +1 880
1-888 = +1 881
1-877 = +1 882
1-866 = +1 883
1-855 = +1 884
1-844 = +1 885
1-833 = +1 886
1-822 = +1 887

This is according to Bellcore, a former administrator of the 
NANP.  The codes on the left are the ones 
reserved for domestic toll-free services, and the ones on the right are 
reserved for the corresponding international
semi-toll-free services.  I use "semi-toll-free" to mean that the US
charges are paid by the callee, while the intl charges are paid by the
caller.

OUT OF DATE INFORMATION FOLLOWS

(c) 1998 Jared Mauch