d/lloodd11_12

mbers which may be screened. Once this is
 done, the cusomter hangs up and the ESS takes over. Now, whenever some
 one calls this particular customer, the customer's switch will compare
 the calling line's directory number with those stored in scratch pad
 memory. If the CLID matches one of the numbers in 1A memory associated with
 the called directory number, the number is forwarded. If not, the phone will
 ring at the original destination. This in particular could make it very
 difficult on system hackers, as you could probably imagine. A company can
 subscribe to this CLASS feature, and enter only the numbers of authorized
 users to be forwarded to a computer. Bureaus inside the various telephone
 companies and other sensitive operations can screen calls to particular
 numbers by using this service.

    This is a security that's hard to beat, but of course there is a way
 (simple law of nature: nothing is fail-safe). There will always be the
 obvious way of finding numbers which ar