Information Technology Reference
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Example 6-18
Transforming Dialed Numbers Using num-exp
Voice_RTR(config)# voice-port 1/0/1
Voice_RTR(config-voiceport)# connection plar 0
Voice_RTR(config-voiceport)# exit
Voice_RTR(config)# num-exp 0 5000
Now, anytime the number 0 is dialed from anywhere in the organization (could be an IP
phone, FXS port, and so on), the voice router automatically transforms it to 5000 and then
searches for a dial peer allowing it to reach the number 5000.
Note: The router applies the num-exp command the instant it receives a dialed number,
even before it attempts to match an inbound dial peer.
Practical Scenario 3: Specific POTS Lines for Emergency Calls
As organizations move more to VoIP connections, they are finding cost-saving benefits
by eliminating traditional telephony connections at remote offices in favor of centralizing
all PSTN calls (and toll charges) at a central site. Figure 6-14 illustrates this type of net-
work design.
This type of call routing allows an organization to get higher call volume from a single lo-
cation, which typically allows the organization to negotiate cheaper long-distance rates
with its PSTN carrier.
Note: Some countries restrict businesses from forwarding PSTN calls over the IP WAN.
You should always check with the local government regulations before you do this.
Thankfully, the United States is not one of those countries.
Although the centralization of PSTN calls offers significant cost savings, the remote sites
need to keep at least one local PSTN connection for emergency calling. This is because
PSTN carriers provide location information to emergency service providers based on the
POTS connection. If emergency calls from the remote offices were to traverse the IP
WAN and leave the PSTN connection at the central site, the emergency service provider
would receive location information for the central site.
Depending on the size of the remote office, you can typically dedicate one or two analog
FXO ports for emergency calls. The configuration in Example 6-19 configures the neces-
sary dial peers for dual FXO ports connected to the PSTN. This example assumes the
FXO ports are 1/0/0 and 1/0/1.
Example 6-19
Dynamic WAN to PSTN Failover Implementation
REMOTE_RTR(config)# dial-peer voice 10 pots
REMOTE_RTR(config-dial-peer)# destination-pattern 911
REMOTE_RTR(config-dial-peer)# port 1/0/0
REMOTE_RTR(config-dial-peer)# no digit-strip
REMOTE_RTR(config-dial-peer)# exit
 
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