Dial Plan Overview
Although most people are not acquainted with dial plans by name, they use them daily. A
dial plan is a numbering plan for the voice-enabled network. It is the way you assign individual
or blocks of telephone numbers (E.164 addresses) to physical lines or circuits. The
North American telephone network is based on a 10-digit dial plan consisting of 3-digit
area codes and 7-digit telephone numbers, as shown in Figure 6-1. For telephone numbers
located within an area code, a 7-digit dial plan is used for the Public Switched Telephone
Network (PSTN). Features within a telephone switch support a custom 5-digit dial plan
for specific customers that subscribe to that service. PBXs also support variable-length
dial plans, containing from 3 through 11 digits. The N in the NXX pattern used for the
area code and the local exchange prefix refers to digits in the 2 to 9 range, whereas an X
represents digits in the 0 to 9 range. Therefore, the first number of an area code or a local
exchange prefix cannot be a 0 or a 1.
Dial plans in the H.323 network contain specific dialing patterns so users can reach a particular
telephone number. Access codes, area codes, specialized codes, and combinations
of the numbers of digits dialed are all part of any particular dial plan. Dial plans used
with voice-capable routers essentially describe the process of determining which digits
and how many to store in each of the configurations. If the dialed digits match the number
and patterns, the call is processed for forwarding.
Figure 6-1 North American Numbering Plan (NANP)
Designing dial plans requires knowledge of the network topology, current telephone
number dialing patterns, proposed router/gateway locations, and traffic routing requirements.
No standard protocol is defined for the dynamic routing of E.164 telephony
addresses. H.323 Voice over IP (VoIP) dial plans are statically configured and managed on
gateway and gatekeeper platforms.
A dial plan consists of the following components:
■ Endpoint addressing (Numbering Plan): Assigning directory numbers to all
endpoints (such as IP phones, fax machines, and analog phones) and applications
(such as voice-mail systems, auto attendants, and conferencing systems) enables you
to access internal and external destinations.
■ Call routing and path selection: Depending on the calling device, you can select different
paths to reach the same destination. Moreover, you can use a secondary path
when the primary path is not available. For example, a call can be transparently
rerouted over the PSTN during an IP WAN failure.
■ Digit manipulation: In some cases, you need to manipulate the dialed string before
routing a call—for example, when a call originally dialed using the on-net access
code is rerouted over the PSTN, or when an abbreviated code (such as 0 for the operator)
is expanded to an extension. This can occur prior to or after a routing decision
has been made.
■ Calling privileges: You can assign different groups of devices to different classes of
service by granting or denying access to certain destinations. For example, you
might allow lobby phones to reach only internal and local PSTN destinations, whereas
executive phones could have unrestricted PSTN access.
■ Call coverage: You can create special groups of devices to handle incoming calls for
a certain service according to different rules (top-down, circular hunt, longest idle, or
broadcast). This also ensures that calls are not dropped without being answered.
322 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
North American Numbering Plan (NANP)
–
10-Digit Dial Plan
N X X N X X – X X X X
5 1 2 – 5 5 5 – 0 1 0 1
User Dials “512-555-0101” Subscriber
512-555-0101
Area Code
Remote
PSTN
Local PSTN
Local
Exchange (CO)
Both Cisco Unified Communications Manager and Cisco IOS gateways, including Cisco
Unified Communications Manager Express and Survivable Remote Site Telephony
(SRST), support all dial plan components.
Table 6-1 compares the methods that Cisco Unified Communications Manager and Cisco
IOS gateways use to implement dial plans.
Table 6-1 Dial Plan Components on Cisco IOS Gateways and Cisco Unified
Communications Manager
Cisco Unified Communications
Dial Plan Component Cisco IOS Gateway Manager
Endpoint addressing POTS dial peers for FXS ports Directory Number (DN)
and ephone-dn if using
UCME/SRST
Call routing and Dial peers Route patterns, route groups, route
path selection lists, translation patterns, partitions,
and calling search spaces
Digit manipulation Voice translation profiles Translation patterns, route patterns,
prefix, digit-strip, forward- and route lists
digits, and num-exp commands
Calling privileges Class of Restriction (COR) Partitions, calling search spaces,
and COR lists and FACs
Call coverage Dial peers, hunt groups, and Line groups, hunt lists, and hunt
call applications pilots
Figure 6-2 shows a typical dial plan scenario with various dial plan components being
deployed. Calls can either be routed via an IP WAN link or a PSTN link, and routing
should work for inbound and outbound PSTN calls, intrasite calls, and intersite calls.
Planning Considerations
The dial plan is the most fundamental attribute of a telephony system. It is at the very
core of the user experience because it defines the rules that govern how a user reaches
any destination. These rules include the following:
■ Extension dialing: How many digits must be dialed to reach an extension on the
system
■ Extension addressing: How many digits are used to identify extensions
■ Dialing privileges: Allowing or not allowing certain types of calls
■ Path selection: For example, using the IP network for on-net calls or using one carrier
for local PSTN calls and another for international calls
Figure 6-2 Dial Plan Example
■ Automated selection of alternate paths in case of network congestion: For example,
using the local carrier for international calls if the preferred international carrier
cannot handle the call
■ Blocking certain numbers: For example, pay-per-minute calls
■ Transformation of the called number: For example, retaining only the last five digits
of a call dialed as a ten-digit number
■ Transformation of the calling number: For example, replacing a caller’s extension
with the office’s main number when calling the PSTN
A dial plan suitable for an IP telephony system is not fundamentally different from a dial
plan designed for a traditional Time Division Marketing (TDM) telephony system.
However, an IP-based system presents the dial plan architect with some new possibilities.
For example, because of the flexibility of IP-based technology, telephony users in separate
sites who used to be served by different, independent TDM systems can now be
included in one unified IP-based system. These new possibilities afforded by IP-based
systems require some rethinking of the way we look at dial plans. This section examines
some of the elements the system planner must consider to properly establish the requirements
that drive the design of the dial plan.
San Jose
DID: +14085552XXX Austin
DID: +15125553XXX
Router2
CUCME
V
CUCME = Cisco Unified Communications Manager Express