DIDs for CER e911

Unanswered Question
Jan 14th, 2009

We are planning a CER implementation. The documentation states that we need at least one DID for each ERL. However, management thinks it is a short-coming of the CER system if it forces such a requirement. I explained to them about how call-back needs a second DID so that CER can intercept the call and ensure that it gets delivered to the phone with no avail. In our scenario, hundreds of rooms are individual ERLs, so this is a bit of an issue.

Is it at all possible to:

1. Just use the phone's DID as the ELIN for some ERLs, with the understanding that call-back may not work if the phone is in DND mode.

2. Have a CallBack number that differs from the ELIN in the ALI database?

Thanks for your assistance.

I have this problem too.
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michael.deal Wed, 01/14/2009 - 08:37

1. It is possible to use the DID for the ELIN, you don't even need CER for this. The problem that CER attempts to solve is the need to update the ERL when that DID moves, however. By using the DID, you have effectively removed the need for CER (except for updates).

2. I'm not completely versed in the structure of the database, but I don't believe that it allows for an alternate callback number.

You may be able to utilize the auto-update features with CER 7 and the proper level of service from Intrado (as long as they hook into your local PSAP). But be certain that you understand the SLAs for location updates in that scenario.

drubery Wed, 01/14/2009 - 09:46

You do not need two DID per ERL, two DIDs simply permit PSAP Callback to multiple users. If you have tow simultaneous calls to 911 and they both disconnect, 911 can call back both extensions. CER caches the extension information on who called 911 for 3 hours for callback, so having two numbers per zone is also helpful.

Aaron Dhiman Wed, 01/14/2009 - 10:18

Sorry, I could have made my posting more clear initially. We definitely need e911. The problem is that we have 400 rooms that each need to be an ERL so we can identify the 911 call down to the room level. For those 400 rooms, we do not want to assign a second DID to be the ELIN.

The simple question then is: For any given ERL, will CER allow the phone DID to be the same as the ELIN?


drubery Wed, 01/14/2009 - 15:45

No, you cannot. Look at the CER documentation for callback and the 913 Route Point. The numbers need to be consecutive so you can establish a route pattern to handle inbound calls, handing them to CER so CER can hand them to the proper extension. If you use DIDs for each extension when someone other than 911 calls in the inbound digits still match the pattern for the 913 Route Point sending the call to CER and not ringing the extension.

drubery Wed, 01/14/2009 - 15:55

I think I said RP for PSAP call backs, I believe it is a Translation Pattern for that range of numbers.

Aaron Dhiman Thu, 01/15/2009 - 09:21

Michael, thanks for this insightful post. Since management is insisting that they do not want to purchase new DIDs, I will configure the ELIN for each ERL to equal the DID of the phone that is there. I have clearly caveated to them that they will need to update their ERL mappings, the Route Pattern for the ERL, and the ALI database to the Service Provider if they choose to move a phone or change a phone number if they choose to stick with this design.

rob.huffman Thu, 01/15/2009 - 06:47

Hi Aaron,

I think the problem here is the question regarding the need for 400 individual ERL's. This is probaly not required (see below), but if it is a "must have" then you will need that many more DID's :)

Here is some info on the relationship between DID's and ERL's;

Please note these two statements "*** The DIDs must be unique for each ERL."

"You can include more specific information using the Phone Location field for individual switch ports."

Emergency Responder converts a caller's extension to an emergency location identification number (ELIN), and this number is used to route the call and to enable PSAP callbacks.

To set up the ELIN numbers, you must first obtain direct inward dial (DID) numbers from your service provider. Because you must pay for each number, you might want to limit the number of DIDs you obtain to two or three per ERL.*** The DIDs must be unique for each ERL.

An emergency response location (ERL) is a building, area within a building, or outside area (if you extend phone service outdoors) that is to be treated as a single location for emergency response purposes. All telephones within the ERL are treated as coming from the same location.

Thus, when someone makes an emergency call, the public safety answering point (PSAP) and your onsite alert (security) team are notified of the ERL. If the emergency requires locating the individual who placed the emergency call, the response teams will have to find the person within the ERL. You can include more specific information using the Phone Location field for individual switch ports. This level of detail is only available for automatically tracked phones, and only appears on the Web Alert screen for onsite alert personnel.

This is similar to the way emergency calls are handled for individual home users: emergency response teams know the house from which the call was placed, but have to search from room to room until they find the caller. The bigger the house, the longer the potential search. Likewise, the larger you make your ERLs, the longer it might take a response team to find an emergency caller.

The laws relating to size of ERLs can vary for different cities, states, and countries. You are responsible for learning your local statutes and developing ERLs that satisfy those statutes. Work with your telephone service provider; they can help you understand the laws. Ultimately, you will have to submit the automatic location information (ALI) for your ERLs to your service provider so that calls from your ERLs are routed to the appropriate PSAPs.

Here are some examples of possible ERLs:

•You have a 25-story building, each floor has 10,000 square feet of office space. You might create 25 ERLs, one per floor. Better, you could divide each floor in half and create 50 ERLs, two per floor.

•You have 5 buildings. Each building was a former home, and they are approximately 3000 square feet. You might create 5 ERLs, one per building, even though some of the buildings are multi-story.

•You have a 5 story building, but the building is very large, so that each floor has 100,000 square feet of office space. You might create 20 ERLs per floor for a total of 100 ERLs, each ERL covering approximately 5,000 square feet.

•You have a high concentration of telephones, and local standards require that an ERL have no more than 48 telephones. In this case, you will have to define zones based on telephone coverage, rather than on physical space. Try to create zones that are recognizable as a physical location, for example, BldJFloor5Row3.

From this doc;


Hope this helps!


JASON PENNELL Wed, 09/15/2010 - 15:14

I'm running into the same type of situation and I have the same question.  How did this install work out?  Did you use a single DID which was assigned to a user and also to an ELIN or did you have to purchase a whole other range of DIDs to be used for the ELINs?  How did your solution work out for you?



drubery Thu, 09/16/2010 - 05:04

As Rob accurately stated you do not need an ELIN for every phone.  PSAP call back to the caller that placed the original emergencey call is facilitated by CER.   CER routess calls into the ELIN DID back to the phone that placed an emergencey call within the last three hours.  If no calls were placed from that ELIN the past three hours it routes the call to the Alert Contact for that ELIN as configure din CER.

Area of a building as defined by your jusrisdiction gets 1-2 or 3 ELINs assigned to it.  CER locates phones by communicating with CallManager and using either subnets or ports.  A caller in the ELIN (Zone) places an Emergencey call, CER intercepts the emergencey call and outpulses to the PSAP one of the ELINs for the zone where the caller resides.  CER logs what extension in what ELIN placed the call.  If the PSAP calls back on that ELIN the call is routed to the extension of the phone that placed the call in the last three hours.

Aaron Dhiman Thu, 09/16/2010 - 13:42

Legally speaking we did not need a phone number for every phone, but it was a School System, so they wanted one for each ClassRoom.  In the end, they eased the requirement, and we segmented the building into larger areas for the ERLs, and they were OK with it.  We additionally have used the Phone Location field (pulled from switch port description field) to provide additional information to Onsite email Alerts.

Good luck with your deployment!

dean_perry Tue, 09/21/2010 - 14:05

We are an education institution somewhat in the same boat. We currently using CER 2.03 have 3000+ phones all of which have unique ELIN's as well as ERL's using phone tracking with locations (Room numbers) listed per port. Everything is working as it should and I get updates whenever a phone is moved, missing or added but I have one issue.

I would love to setup zones for our 15 locations to cut down on the manual upkeep of the database as we have people moving devices all the time. The problem is that when a 911 call is made  CER does not send the port locations in email alerts it only sends the location listed in the ALI details per that zone and when logged in as an "ER user" our dispatch cannot see the switchport location field so they cant pinpoint the caller when its setup this way.

I would like to be able to send the port location in the email and have it show up in the call history and alerts under the user ER user login. Is this possible? How hard could it be to forward the port locations in the alerts when a call is made? Am I missing something?



Aaron Dhiman Tue, 09/21/2010 - 15:38

Dean, it is interesting that you state that moving devices causes manual upkeep of the database.  Are you using Automatic Phone Tracking, or have you manually entered the phones?  With automatic phone tracking, no manual intervention is required for a phone move.  What exactly do you have to do?

In terms of the Alerts, you should get the information from the Location in the email as well, even with CER 2.0.  From the 2.0 Admin Guide:

If you configure email addresses for the personnel, they also receive an email, which includes

more information than the phone call, including ERL name and phone location. If the email

address is for an email-based pager, they are paged. Paging is the most efficient way of getting

information to users who are not at their desks.

dean_perry Wed, 09/22/2010 - 09:39


Thanks for the response. In our environment we need to know exact room numbers from where the call is being made and and want the callback # to be from that specific device so the way it now, each switch port has a specific ERL/RP with specific ALI location information(rm number) such as our Service Center would be SVC-31120, or another campus may be CYP-32240. So, if a phone moves from one port to another I do get a switch-port change email but the ERL on that switch port has to be updated. If I dont update the ERL I get something like this.

11010264911 5122235493 -->
EVC-354933543310:52:20 AM CDTSeptember 16, 201011010264911--XXXXXXX5493

In this case the user moved to a new office that was previously occupied by another user who in turn move to a new campus both of which I got an email via the phone tracking. A 911 call was made from the user with extension (35433)  before the update was made and the call-back is sent for the ERL-35493 (the previous occupant) which would be the wrong callback # although the correct location.

Below is a recent email alert format.The location seen below is taken from the information in the ALI not the actual location listed on the switch port itself.

Subject: Emergency Call Alert -- Extn # 35433

    EMERGENCY CALL DETAILS (Generated by CiscoER)
Caller Extension : 35433
Zone/ERL         : EVC-35493
Location         :  BLD 1000/1110
Call Time        : September 16, 2010 10:52:20 AM CDT

For detailed call information please refer to --

Now, lets say I made a ERL zone (EVC-bldg1000) and assigned it to the switch ports in that building with the ALI location and RP of our campus police. If a 911 call was made from one of those ports the alert would pull the location from the ALI info of the location of our campus police and yes they would have a callback extention but the actual location (rom number) on the switchport is not sent and dispatch cannot look at the switchport with only "ER User" rights so they would have no idea what specific room it was in.

This would not be an issue IF: The actual switchport location was sent in the email alerts or I could create a role that would allow "ER User" to access switchport locations in read only mode. I don't see a way to do this in ver 2.0. Is 7.1 more configurable as I have it otw.

Kinda confusing but that make sense?

Aaron Dhiman Wed, 09/22/2010 - 11:41

Well, this is precisely why ERLs get unique DIDs, which should NOT be Extension Numbers used on the phones!  This is specified here:


This fact is not obvious, but very important.

So, you would need TWO DIDs for each of your rooms.  One is the extension on the phone, and the other is for the ELIN.  For simplicity, let's say:

ERL = EVC-35493

ELIN = 2000

Phone in EVC-35493 has Extension = 1000

1000 makes the 911 call, which CER intercepts.  CER REPLACES 1000 with 2000 for the ANI.  When the call goes out now, 2000 is the callback number, which is also in the ALI Database mapping to EVC-35493.  You can then move any phone in and out of there, and you will never have to make any changes...CER will track phones based on what port they are connected to on the switch.

As for using zones with more detailed description for Phone Location, you can set that with CER 7 or 8 to pull the switch port description from the switch:


"If you want to display the switch port descriptions that is configured on the switch in the locations field in Cisco ER, select Use port description as port location."


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