Examining POTS Dial-Peer Digit Stripping

When a call matches a POTS dial peer, by default the router strips the explicitly matched (nonwild-card) digits on the left. Any remaining digits that match a wildcard or are to the right of a wildcard are not stripped. As an example, we have this dial peer:

dial-peer voice 1 pots destination-pattern 555....

port 2/0/0

If a user dials 5555439, the result is that the 555 is explicitly matched and stripped, while the 5439 matches the wildcard digits and those digits are sent out the port.

There are two main reasons for this default digit-stripping behavior:

■ When constructing a dial plan for a PBX-type environment, an outside access digit (such a dial-ing a 9 to get an outside line) is needed, or it would be very difficult to dial any number on the PSTN that had the same first digits as your internal extensions, as the calls will complete when the last digit of an extension is dialed. (This is rule 2 of dial-peer selection in Step 1-1.) When using an outside line access digit, this number should not be sent to the PSTN and needs to be stripped.

■ When the POTS dial peer points to an FXS port, the analog device has no need of the dialed digits because it is an endpoint. (In contrast, for a digital trunk that connects to the PSTN, the dialed digits are needed to route the call through the PSTN.) (Just for fun, if you have an FXS port, create a dial peer with a destination pattern of seven digits or more, use one of the follow-ing digit manipulation commands to keep all the digits, and connect an analog phone to the FXS port. Then dial the number in the destination pattern from an IP Phone. When you answer the ringing analog phone, you will hear the unnecessary dialed digits.)

However, in many cases, we don’t want all the digits that are explicitly matched to be stripped. By using digit manipulation commands, you can change the default digit-stripping behavior of POTS dial peers. Table 6-9 shows the common digit manipulations used.

Table 6-9 Common Digit Manipulation Methods on Cisco Routers

Command Description

Example

no digit-strip Turns off digit stripping for a POTS dial peer.

Advantage: No digits are stripped.

Disadvantage: Outside line access digit is not stripped.

To send a specific seven-digit number range to the PSTN:

dial-peer voice 3 pots destination-pattern 9555....

no digit-strip port 2/0/0

If 95551234 is dialed, 95551234 is sent on.

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Command Description

forward-digits [all | x] The forward-digits all command acts just like no digit strip. The for-ward-digits x command on a POTS dial peer sends on just the x number of digits in the destination pattern from the right.

Advantage: The forward-digits x command controls the exact number of digits to be saved; thus the outside line access digit can be stripped while all other digits are sent on.

Disadvantage: This is one of the few commands that cannot be removed with the no option. To remove the command, use default forward-digits.

To send a seven-digit number to the PSTN when a specific ten-digit number range is dialed and 9 is the outside access digit:

dial-peer voice 6 pots

destination-pattern 91510555....

forward-digits 7 port 2/0/0

If 915105551234 is dialed, 5551234 is sent on.

prefix digits Adds the specified digits to the left, after digit stripping happens in the POTS dial peer. Because the digits to be stripped are known, it is easy to prefix them back while ignoring the outside line access digit. The prefix command can also add digits that were not dialed to the start of the dialed digits.

Advantage: Can not only replace stripped digits, but also adds additional new digits to the left of the dialed digits. It can also add a one-second delay for every comma (,) added to allow automated systems to answer.

Disadvantages: None.

To send a long-distance digit number (1+10 digits) to the PSTN when a specific seven-digit number range is dialed and 9 is the outside access digit:

dial-peer voice 4 pots destination-pattern 9556....

prefix 1510556 port 2/0/0

If 95561234 is dialed, 15105561234 is sent on.

Use a wildcard for the By using a wildcard for the first digit to be saved in the POTS destination first digit (or all the digits) pattern, the other remaining digits are not stripped. The wildcard can be to be saved. as simple as putting a single digit in brackets, for example, [5]55...

Advantage: No other method is required to save the digits.

Disadvantage: None.

To send the three-digit North American N11 codes (more info at

www.nanpa.com/number_resource_info/n11_codes.html) and 9 is the outside access digit:

dial-peer voice 8 pots destination-pattern 9[2-9]11 port 2/0/0

If 9311 is dialed, 311 is sent on.

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Command Description

num-exp match_digits Number expansion globally transforms any digits matching the expanded_digits match_digits pattern using the expanded_digits. There can be more

replaced digits than the digits matched.

Advantage: Transforms digits globally so that they don’t need to be applied to every dial peer or ephone-dn.

Disadvantage: Transforms digits globally, so you are unable to control which dial peers use the transform.

When callers dial 0 for operator services, redirect the call to an internal support desk at extension 1234:

num-exp 0 3344

If 0 is dialed, the digits are changed to 3344.

Voice translation profiles Voice translation profiles allow extremely complex digit manipulation.

They can change both the dialed and calling numbers, and are for calls inbound, outbound, or in both directions. Regular expression concepts can be used to save some digits while changing others.

Advantage: Extremely powerful and can be applied to individual dial peers or ephone-dns. Can also be used in VoIP dial peers to change digits.

Disadvantage: Can be tricky to understand and troubleshoot.

When callers dial 911 for emergency services, redirect the call to an internal security service at exten-sion 1234:

If 911 or 9911 is dialed, the digits are changed to 1234.

Note: The details of implementing number expansion and voice translation profiles are not a focus of the CCNA Voice Certification exam and are not covered further in this lab book.

To compare the various digit manipulations, examine a dial peer for emergency services (the number is 911 in North America; elsewhere 112 is the most common). If nothing is done, a destination pattern 911 in a POTS dial peer would have all the digits matched; thus all digits are stripped and no digits are sent to the PSTN. The various methods that are commonly used to resolve this are as follows:

destination-pattern 911 no digit-strip

or

destination-pattern 911 forward digits all

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Note: Typically, you use only one method per dial peer. If you mix more than one method on the same dial peer, it can cause unexpected results.

However, if an outside access digit (such as dialing a 9 to get an outside line) is used, some of the pre-vious methods will not work, as they also send the access digit, which causes the PSTN to reject the call. The various methods that are commonly used to resolve this are as follows:

destination-pattern 9911 forward digits 3

or

destination-pattern 9911 prefix 911

Note: From a legal and regulatory standpoint, access to emergency service numbers is one of the most critical functions of the phone system. This can cause design issues with your dial plan. To avoid accidental calls (and people hanging up on emergency operators), the approach you take can vary. One of my students worked at a company that changed its outside access line digit from 9 to 8 just to avoid accidental 911 calls. At Madison College where I work, the college does not support dialing 911, only 9911. As a result, for legal reasons, the col-lege needed to notify everyone that 911 alone would not work. To comply, all phones show “Call 9-911 for emer-gency services” off to the side of the college logo image on the phone displays. Cisco also sells a server product, Cisco Emergency Responder (CER), that offers enhanced ability to track phone locations for emergency response compliance, as well as other functions.