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Calling privileges control the available components of a call-routing database that are
accessible to an endpoint. The primary application is the implementation of CoS. CoS
is usually used to control telephony charges by blocking costly service numbers. Many
organizations block international calls for most users and restrict long-distance dialing
on common-area phones. CoS is also used to protect the privacy of some users. Executive
managers may allow only those calls that have gone through their assistants, for example.
Calling privileges can also be used to implement special applications such as tail-end hop
off (TEHO). TEHO allows organizations to save public switched telephone network (PSTN)
toll charges by routing long-distance and international calls across the private IP WAN
network before hopping off at the destination-site gateway to route a local PSTN call.
TEHO is an application of least-cost routing (LCR), which has been in telephony networks
for a very long time.
TEHO can greatly complicate a dial plan because of the additional configuration required
to properly route calls on a per-site basis. In a multisite environment with PSTN gateways
at each site, PSTN route patterns should always be routed to the local PSTN gateway; hence,
the same route patterns have to exist multiple times (once per site in this example), and only
the site-specific route patterns should be accessible by the devices located at this site.
Another application is time-of-day routing, where calls should take different paths
depending on the time when the call is placed.
Table 13-1 provides a typical CoS implementation with calling classes and their allowed
destination. These calling classes can then be assigned to devices or users.
In the example, class Internal allows only internal and emergency calls. Class Local adds
the permission for local PSTN calls, class Long Distance also allows long-distance PSTN
calls, and class International also enables international PSTN calls.
Table 13-1 Class of Service Example
Class of Service Allowed Destinations
Internal Internal
Emergency
Local Internal
Emergency
Local PSTN