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Reachability of internal destinations is provided by assigning directory numbers (DN) to all
endpoints (IP phones, fax machines, and analog phones) and applications (voice mail, auto
attendants, and conferencing systems).
The number of dialable extensions determines the quantity of digits needed to dial extensions. A
four-digit abbreviated dial plan cannot accommodate more than 10,000 extensions (from 0000 to
252 Chapter 11: Call Routing Components
9999). A leading digit of 0 is normally not used in dial plans because it is dedicated to
operator functionality. A leading digit of an 8 or 9 is also normally restricted because either
one of these digits is used to make outgoing calls to the public switched telephone network
(PSTN). If we assume access code 9 is used, the access code and operator functionality
reduce the number range to 8000 dialable patterns (1000 to 8999). Customers may want to
distinguish each site by a different leading digit, too. This would reduce the scalability of
the system to eight sites if each site would receive a different leading digit (1 to 8). Each
site could have 1000 directory numbers (1000 to 1999 as an example). A four-digit dial is
not very popular with large organizations. Most organizations use at least a five-digit dial
plan.
If direct inward dial (DID) is enabled for incoming PSTN calls, PSTN phone numbers
directly dial internal directory numbers. This restricts the internal directory numbers used
by the DID range purchased from the PSTN provider.
Endpoints dial each other using the four- or five-digit dial plan that was assigned to the
phones. Each call made by a Cisco IP Phone is categorized as one of the following:
■ On-net: These are all calls that remain within one telephony system or IP network (an
internal call from one IP phone to another IP phone). These calls may involve multiple
CUCM clusters. The administrators can control how calls made between CUCM
clusters are categorized.
■ Off-net dialing: These are calls that are placed from one telephony system to another
telephony system. Most off-net calls are routed across gateways or trunks to the PSTN.
■ Abbreviated dialing: This is when an off-net destination is dialed by an internal
number (for example, dialing a four-digit extension to reach colleagues on their home
office PSTN number). CUCM has to map the abbreviated number to the appropriate
full PSTN number in this case.
In Figure 11-1, an IP phone located in the headquarters (extension number 2003) dials 3001
to reach an IP phone located at Site 1 over the IP WAN. Both devices are part of the same
CUCM cluster. No toll charges are incurred for the call, and the call is classified as an onnet
call.
The IP phone with extension 2002 dials 95552001, and the call is routed to a PSTN
destination through a PSTN gateway. The call is classified as an off-net call.
The IP phone with extension 2001 dials 4001, which is an IP phone located at Site 2. CUCM
Express is used for call processing at Site 2, and no IP WAN link connects the sites. Because
Site 2 cannot be reached over an IP WAN and the service provider (PSTN) requires tendigit
dialing to properly route the call, a CUCM translation pattern is matched, and the call
Uniform On-Net Dial Plan Example 253
is manipulated from four-digit dialing to ten-digit dialing. Translation patterns are covered
in more detail in the next chapter.