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When you implement PSTN access in a multisite environment, you must perform digit
manipulation as described in the following list before the call is sent to the PSTN. Digit
manipulation has to be done in CUCM when you use an MGCP gateway. It can be
performed either in CUCM or at the H.323 gateway when using an H.323 gateway.
• Outgoing calls to the PSTN:
—Automatic Number Identification (ANI) transformation: If no direct
inward dialing (DID) range is used at the PSTN, transform all directory
numbers to a single PSTN number in the ANI. If DID is used, extend the
directory numbers to a full PSTN number.
—DNIS transformation: Strip the access code.
• Incoming calls from the PSTN:
—ANI transformation: Transform ANI into the full number (considering
Type of Number [TON]), and add the access code so that users can easily
redial the number.
—DNIS transformation: If DID is used, strip the office code, area code,
and country code (if present) to get to the directory number. If DID is not
used, route the call to an attendant such as a receptionist or an interactive
voice response (IVR) application.
Figure 4-4 shows an example of digit manipulation performed for both incoming and
outgoing PSTN calls.
As shown in the figure, internal numbers have to be represented as valid PSTN numbers,
and PSTN numbers should be shown with access code 9 internally. Recall from Chapter 1,
"Identifying Issues in a Multisite Deployment," that the ANI is the number calling from,
and the DNIS is the number calling to. Note how four-digit dialing is used internal to
CUCM in this example because incoming DNISs are manipulated to four digits.
Chapter 4: Implementing a Dial Plan for Multisite Deployments
Figure 4-4 PSTN Access Example
NOTE Adding the access code (and changing ten-digit PSTN numbers to 11-digit
PSTN numbers, including the long-distance 1 digit) to the ANI of incoming calls is not
required. Adding it, however, allows users to call back the number from call lists (such
as received calls or missed calls) without having to edit the number by adding the
required access code.
Transformation of Incoming Calls Using ISDN TON
The TON is used to specify in which format a number such as ANI or DNIS is represented.
To have a unique, standardized way to represent PSTN numbers in CUCM, the numbers
have to be transformed based on the TON.
U.S. TON in ISDN provides information about number format:
• Subscriber
Seven-digit subscriber number: three-digit exchange code, four-digit station code
• National
Implementing PSTN Access
Ten-digit number: three-digit area code, seven-digit subscriber number
• International
Variable length (11 digits for U.S. numbers):
—Country code (one digit for U.S. country code; one, two, or three digits
for all other countries)
—Area code (three digits for U.S. area code)
—Subscriber number (seven digits for U.S. subscriber number)
NOTE This description is based on the North American Numbering Plan (NANP),
which applies to the United States, Canada, and several Caribbean nations, as described
at http://www.nanpa.com.
Figure 4-5 shows an example of performing TON-based digit manipulation based on the
incoming call's ANI.
Figure 4-5 ISDN TON; ANI Transformation of an Incoming Call
Site 1
1001-1099
Site TON ANI Required ANI Transformation
1 Subscriber 5551111 9.5551111
2 National 7145552222 9.1714555 2222
3 International 49404132673333 9.01149404132673333
86 Chapter 4: Implementing a Dial Plan for Multisite Deployments
NOTE This example demonstrates the commonly used access code 9 to dial out to the
PSTN. It is perfectly acceptable for an organization to choose another access code such
as 8, or no access code at all. The Required ANI Transformation digits in this example
would be changed accordingly.
Implementing Selective PSTN Breakout
A multisite deployment, shown in Figure 4-6, typically has multiple PSTN gateways, usually
one per site. Selective PSTN breakout ensures that local gateways are used to access the
PSTN. From a dial plan perspective, this can be achieved by creating one 9.@ route pattern
if you're using the North American Numbering Plan; otherwise, use 9.! per site. These route
patterns are put into different partitions and point to different PSTN gateways. IP Phones
need to be configured with a CSS that includes only the route patterns that refer to the local
gateway. This way, IP Phones will always use their local PSTN gateway for PSTN breakout.
Figure 4-6 Configure IP Phones to Use a Local PSTN Gateway
Main Site
In this figure, three different calls are received at the main site gateway. The first call is
received from the local area with a subscriber TON and a seven-digit number. This number
only needs to be prefixed with access code 9. The second call, received with national TON
and ten digits, is modified by adding access code 9 and the long-distance 1, both of which
are required for placing calls back to the source of the call. The third call is received from
another country (Germany in this case) with an international TON. For this call, the access
codes 9 and 011 have to be added to the received number, which begins with the country
code of 49. Note that 011 is the NANP international access code, which is different for calls
originating outside the NANP.
The end result benefits an internal user who receives but misses any calls from these sites
and wants to easily call back any of these numbers without editing the number from his or
her missed call list.