Well that's a good question.I remember reading something about that and this is what I came up with.
This is iDCS500 Release 1 documentation
software release 1.31

MMC 765 CID Reject
When an incoming call with CID hits the switch,if the CID number is compared and a match is found in the pre assigned table,the CID number will be rejected by the system.This feature is not recommended for use in North America.

This only appears in the software update and does not appear in the release 1 or 2 manuals that I have.Tha's all the info I have on it.