| Calling-line identification screens
incoming ISDN calls. The called number supplied in the call message
setup request is verified against a table of allowed numbers. This
feature prevents charges for calls from unauthorized numbers. In
some situations, there are charges for call setup attempts, so even
if the call does not pass caller ID screening, there is a charge for
the attempt.
Caller ID is available only from providers and switches that supply called number values during the call setup phase. As shown in the Figure, call acceptance does not occur until the router verifies the calling number.
Caller ID screening requires a local switch that is capable of delivering the caller ID to the router or access server. If you enable caller ID screening, but do not have such a switch, no calls are allowed in. In addition, caller ID screening records the number exactly as it was sent, with or without an area code prefix.
The isdn caller command can be used to configure ISDN caller ID screening. The number is a telephone number, up to 25 characters in length. As part of this number, enter an x or X in any position in the number where you can accept any number as a match (a so-called
wildcard or
don’t-care digit). You can specify up to 64 numbers per interface.
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