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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.
Implementing Selective PSTN Breakout
PSTN breakout occurs at the gateway located closest to the PSTN destination:
• Route patterns for each area that can be reached at different costs—one per site, in
different partitions
• Route patterns point to route lists (with different priorities of gateways: cheapest
gateway first, local gateway next, and then another gateway for additional backup if
desired).
• Phone CSS for correct route-pattern selection
NOTE If greater control over restricting outbound dialing is required, more-specific
route patterns should be created instead of those using the generic @ wildcard.
Configure IP Phones to Use Remote Gateways for
Backup PSTN Access
Figure 4-7 shows how a multisite dial plan can feature multiple choices for PSTN access to
provide a backup in case of primary (local) gateway failure.
Figure 4-7 Configure IP Phones to Use Remote Gateways for Backup PSTN Access
Main Site
Primary Path Backup Path
In this figure, both sites have IP Phones and a PSTN gateway. As discussed earlier in this
book, IP Phones should always use their local gateway to access the PSTN, which is critical
for emergency 911 calls. A user dialing 911 in Atlanta certainly wouldn't want to reach the
emergency call center in Chicago. This example of geographic routing can be achieved
by using multiple route patterns in partitions and CSSs at the IP Phones. If the local PSTN
gateway has a failure while the WAN link is functional, you can provide backup PSTN access
by using route lists and route groups with multiple gateways. Because a route list and route
group is a prioritized list of devices, it is simple to specify the primary and backup path.
Chapter 4: Implementing a Dial Plan for Multisite Deployments
• Use the secondary gateway's PSTN number.
When the ANI of the backup gateway is used, the called party may get confused about
the number that should be used when calling back. For instance, the person may update
his address books with the different number and inadvertently end up sending calls
through the backup gateway every time he calls. Further DID ranges would have to
include remote phones or IVR scripts (automated attendants) to be able to route calls
to phones located in any site, regardless of where the PSTN call was received.
CAUTION Using a remote gateway for PSTN access might not be permitted in your
country or by your provider. There can also be issues with emergency calls. Therefore,
ensure that your planned deployment complies with legal requirements.
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