CUCM call routing (Part 2): Dial tones and route patterns

In the last blog, we began analyzing an example PSTN dial-plan. The dial plan has been provided below to make it easier to cross reference the explanation that follows:

• 911 Emergency Call Routing

• 9.911 Emergency Call Routing w/access code

• 9.[2-8]11 Three digit service codes

• 9.[2-9]XXXXXX Local 7-digit dialing

• 9.[2-9]XX[2-9]XXXXXX Local area code 10-digit dialing

• 9.1[2-9]XX[2-9]XXXXXX Long Distance 11-digit dialing

• 9.011! International dialing

• 9.011!# International dialing

• 9.0 PSTN operator Most organizations have, and continue to use, a 9 to access an outside line.

When a 9 is dialed, a secondary dial tone (stutter tone) is heard. Some legacy PBX (private branch exchange) systems would route calls directly to a trunk card when the 9 access code was dialed. This resulted in a secondary dial tone from the PSTN provider. In the effort of maintaining a similar end user experience, Cisco Unified Communications Manager (CUCM) is capable of providing stutter dial tone.

The Provide Outside Dial Tone capability is selected by default in all CUCM route patterns. The Call Classification defaults to Offnet when this option is selected. The dot (“.”) is used as a delimiter for digit stripping. A DDI (Digit Discard Instruction) rule will be appled to the called party (DNIS) that will strip the 9 before forwarding the call to the PSTN (Pre-Dot). The placement of the dot becomes critical in complex call routing scenario. We’ll keep it simple for now.

The 9.[2-8]11 route pattern will be used to route all 3 digit service codes that begin with any one digit (2-8) followed by a 11. 411 is typically used for information services and 511 is used for local city information, but the use of 511 and 311 vary by city. Notice that the wild card range does not include a 9. The 9 is not necessary because there is an explicit match for 9.911 with a separate route pattern. The provisioning of 9.[2-9]11 would not affect the 9.911 route pattern call routing. The 9.[2-9]11 route pattern would never be selected for the dialed digits of 9911 because 9.911 is an exact match. The 9.[2-9]XXXXXX route pattern is used for local 7-digit dialing.

If you live in a major metropolitan area, 7 digit dialing is probably no longer allowed. In New York City, you have to dial 10-digits to call a pizza parlor across the street. If you live in a rural area like I do, 7-digit dialing is still alive and well. Let’s examine 7-digit dialing for a moment. 7-digit dialing is comprised of two pars. The first three digits represent the NXX (exchange). Each exchange represents a different geographical area where a CO (central office) is located.

The next four digits represent the 4-digit subscriber code. This is somewhat analogous to mailing addresses. The 4-digit subscriber code would represent a house number and the 3-digit exchange would represent the city. Notice that none of the patterns discussed so far have a 0 or 1 as a leading digit after the 9 access code. The designers of the NANP (North American Numbering Plan) did this on purpose to avoid an overlapping dial plan. A leading digit of 0 represents the local PSTN operator (International operator in the case of 9.011).

A leading digit of 1 represents long-distance dialing. The T.302 inter-digit timeout is your enemy. Avoid overlapping dial plans at all costs. The designers of the NANP understood this. We will continue this discussion of dial plan in the next blog entry. I’m sure you can hardly contain your excitement at this point and you are probably thirsty for some more NANP knowledge… maybe not. Search the term “NANP” on for an interesting article on the history, present, and future of the NANP. The developers of the NANP did not anticipate every human being having a personal cell phone number, work cell phone number, office DID number, home phone number, and home fax line.

Please ask questions, make comments, or collaborate in any way you would like.


Copyright © 2008 IDG Communications, Inc.

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