When a terminating router receives a voice call, it selects an outbound POTS dial peer by comparing the called number (the full E.164 telephone number) in the call information with the number configured as the destination pattern in the POTS dial peer. The access server or router then strips off the left-justified digits that match the destination pattern. If you have configured a prefix, the prefix is added to the front of the remaining digits, creating a dial string, which the router then dials. If all numbers in the destination pattern are stripped out, the user receives a dial tone.
For example, consider a voice call whose E.164 called number is 1(408) 555-2222. If you configure a destination-pattern of "1408555" and a prefix of "9," the router strips off "1408555" from the E.164 telephone number, leaving the extension number of "2222." It then appends the prefix, "9," to the front of the remaining numbers, so that the actual numbers dialed are "9, 2222." The comma in this example means that the router will pause for 1 second between dialing the "9" and dialing the "2" to allow for a secondary dial tone.
When the terminating router matches a dial string to an outbound POTS dial peer, by default the router strips off the left-justified digits that explicitly match the destination pattern. Any remaining digits, called> excess digits, are forwarded to the telephony interface, such as a PBX or the PSTN.
Some telephony interfaces require that any digits stripped from the dial string be recovered to support a particular dial plan. You can strip these digits either by using the no digit-strip dial-peer voice configuration command to disable the default digit-stripping behavior or by using the prefix dial-peer voice configuration command to add digits to the beginning of the dial string before it is forwarded to the telephony interface. These commands are supported only in POTS dial peers.
The no digit-strip command disables the automatic digit-stripping function so that matching digits are not stripped from the dialed string before it is passed to the telephony interface. For example, in the following dial peer configuration, the entire seven-digit dialed string is passed to the telephony interface:
dial-peer voice 100 pots destination-pattern 555.... no digit-strip port 1/0:1
Disabling digit stripping is useful when the telephony interface requires the full dialed string. With some dial plans, however, the dialed digits must be manipulated according to specific rules. The prefix command can be used to add specific digits to the beginning of the dialed string before it is forwarded to the telephony interface.
For example, consider a telephone whose E.164 called number is 1(408)555-1234. This telephone can be reached within the company by dialing its extension number, 51234. If you configure a destination pattern of "1408555...." (the periods represent wildcards) for the associated outbound POTS dial peer, the terminating gateway will strip off the digits "1408555" when it receives a call for 1(408)555-1234. For the terminating gateway to forward the call to the appropriate destination, the digit "5" needs to be prepended to the remaining digits. In this case, you would configure a prefix of 5, as shown in the following dial peer configuration.
dial-peer voice 100 pots destination-pattern 1408555.... prefix 5 port 1/0:1
A prefix can also include commas (,). Each comma indicates a 1-second pause in dialing. For example, consider a telephone whose E.164 called number is 1(408)555-1234; to reach this device, you must dial "9." In this case, you might configure "1408......." as the destination pattern, and "9" as the prefix. In this example, the terminating router will strip the digits "1408" from the called number and append the digit "9" to the front of the remaining digits, so that the actual number dialed is" 9,5551234." The router pauses for 1 second between dialing the "9" and the "5551234" to allow for a secondary dial tone. In this example, you would configure the router as follows:
dial-peer voice 100 pots destination-pattern 1408....... prefix 9, port 1/0:1
Using a comma with the prefix command is useful when the router must allow for a secondary dial tone; otherwise the router does not wait for the dial tone before playing out excess digits. Putting commas in the prefix makes the router pause 1 second per comma, allowing for a dial tone to occur before the router transmits the remaining digits.
The figure below shows an example of a network using the no digit-strip command. In this example, a central site (Site D) is connected to remote sites through routers (Sites A, B, and C), and through a Centrex system for sites still using the PSTN (Sites E and F). The Centrex service requires the full 7-digit dial string to complete calls. The dial peers are configured with a fixed-length 7-digit dial plan.
|Figure 2 ||Network with Digit Stripping Disabled or Prefixes Enabled |
When Site E (8204...) dials 8201999, the full 7-digit dialed string is passed through the Centrex to the router at Site D. Router D matches the destination pattern 8201... and forwards the 7-digit dial string to Router A. Router A matches the destination pattern 8201..., strips off the matching 8201, and forwards the remaining 3-digit dial string to the PBX. The PBX matches the correct station and completes the call to the proper extension.
Calls in the reverse direction are handled similarly, but because the Centrex service requires the full 7-digit dial string to complete calls, the POTS dial peer at Router D is configured with digit stripping disabled. Alternatively, digit stripping could be enabled and the dial peer could instead be configured with a 4-digit prefix, in this case 8204, which would result in forwarding the full dial string to the Centrex service.
dial-peer voice 1 pots destination-pattern 8201... port 1/0:1!dial-peer voice 4 vofr destination-pattern 8204... session target s0 2!dial-peer voice 5 vofr destination-pattern 8205... session target s0 2!
dial-peer voice 4 pots destination-pattern 8204... no digit-strip port 1/0:1!dial-peer voice 5 pots destination-pattern 8205... no digit-strip port 1/0:1!dial-peer voice 1 vofr destination-pattern 8201... session target s0 1!