ZyXEL Communications Prestige 28641 - Manual (Page 84)

70 Dial-In Configuration
More on CLID
CLID allows the Prestige to authenticate the caller before a call is
answered, thus saving the cost of a connection. The Prestige uses the
caller ID in the ISDN call setup message to match against the CLID in
the database.
However, CLID may not be available due to your switch configuration.
Besides authentication, another application of CLID is to combine it
with call back. For instance, your company pays for the connection
charges for telecommuting employees, and you are using the Prestige as
the dial in server. You can turn on both the CLID authentication and call
back options for the dial-in users. By doing so, all usage are charged to
the company instead of the employees, and your accounting department
can avoid the hassles of accountability and reimbursement.