9-1-1 and SIP

Have you ever had the need to dial 9-1-1? Unfortunately, I have. It was just one time, but one time is enough for me. I won’t bother you with the details, but minutes after the call was answered, a swarm of paramedics had converged on my house and although the emergency was resolved seconds prior to their arrival, I am grateful that the right people could be summoned so quickly.

According to that National Emergency Number Association (NENA), 240 million 9-1-1 calls are made each year. The FCC adds that one third of those calls are from wireless telephones. In some communities, nearly one half of the 9-1-1 calls are made from cell phones. Clearly, the need for this single number to call for help is extremely valuable.

9-1-1 has been around since 1968, but I highly doubt that most of those 240 million people knew what was happening when they made their calls.

How do the calls end up at the right place? How do the emergency response agents know where the calls are coming from?

Before I delve into explanations, allow me to define a few terms.

NENA. The National Emergency Number Association (NENA) is the organization responsible for fostering the technological advancement, availability, and implementation of an emergency telephone number system in the United States.

PSAP. A Public Safety Answering Point (PSAP) is a call center responsible for answering 9-1-1 calls. As of April 2014, the United States has 5,976 primary and secondary PSAPs in 3,135 counties, parishes, independent cities, boroughs and Census areas.

ELIN. An Emergency Location Identification Number (ELIN) is a North American dialing plan number used to represent one or more telephone extensions off a PBX. The telephones that use the same ELIN should be physically located near one another. ELINs can be used when it is not necessary to identify the exact location of every telephone in a business.

ANI. Automatic Number Identification (ANI) is a ten-digit number sent from a telephone on the public network. Like an ELIN, it identifies the caller.  However, ANI is generally assigned to a more precise location.

ERL. An Emergency Response Location is typically used by a PBX to define telephone locations. A company may have many ERLs, but fewer than the number of employees or DID numbers.  An ERL might be the company address along with information about a location within that address (e.g. southwest corner of the second floor). Several ELINS may be assigned to a single ERL.

Selective Router. Also referred to as a Tandem Router, a Selective Router uses an ANI/ELIN to find the correct PSAP for an emergency call.

MSAG. The Master Street Address Guide database maintains the ELIN to PSAP mapping and is used by the Selective Router.

PS-ALI. The Private Switch Automatic Location Identifier database is used by the PSAP to map ANI/ELINs to caller information. This provides the emergency response agent with the ALI for the caller.

ALI. Automatic Location Identification (ALI) provides the PSAP agent with location information about the 9-1-1 call.  ALI consists of the following:

  • Phone number
  • Location, 20 characters
  • Customer name
  • Street directional
  • Street name
  • Community and state

Whew! Does that sound a little overwhelming? Actually, once you understand the basic call flow, it’s not that bad. To assist you in understanding how it all comes together, let’s take a look at a very simple example.

  1. Someone makes a 9-1-1 call. The caller is identified by his or her ANI or ELIN. ANI if the call is from a telephone on the public network. ELIN if the call is from a private network such as a PBX. Remember, ERLs are used to map locations within a business to  ELINs.
  2. The Telephone Company/Selective Router uses the ANI/ELIN to search the MSAG to determine which PSAP should handle the call.  The call is then sent to that PSAP.
  3. When the call arrives at the PSAP, the ANI/ELIN is passed to the PS-ALI database to determine the correct ALI for this call.
  4. The call is routed to an agent along with the retrieved ALI.
  5. If necessary, emergency response providers are dispatched to the location described within the ALI.

9-1-1 from a cell phone is a little more complicated, but by using cell tower triangulation, the caller’s location can also be determined.

That’s not so bad, is it? As long as the call is made to the appropriate place and the ANI/ELIN matches an entry in the PS-ALI database, the emergency will be efficiently handled.


* Image from Wikipedia.

However, what if the call isn’t sent to the appropriate place or the ALI cannot be determined from the ANI/ELIN? These are the issues that arise from IP telephony.

In the TDM world, telephones are pretty static. You plug your phone into a jack that is directly connected to wire that terminates on a port off your PBX. This makes it pretty easy to match caller to location.

Additionally, the telephones and PBX are in the same location and the PBX has TDM trunks to the local telephone company. So, if a user off a PBX in Bloomington, Minnesota makes a 9-1-1 call, the chances are extremely high that it will be answered at a nearby PSAP.

IP Telephony Brings Issues

SIP phones aren’t physically tied to a PBX. This is especially true with a SIP client on a mobile device. With SIP, I can make calls regardless of where I am. However, it won’t do me any good if I have an emergency in Illinois and the 9-1-1 call is answered by someone in Minnesota. Clearly, something needs to be done to connect callers with the correct emergency responders.

Ignoring the case of mobile or remote SIP for a moment, this is where the ERL can be used. ERLs can be associated with ports on LAN switches. An ERL for one group of ports might map to one set of ELINs and the ERL for another group of ports might map to a different set of ELINs. ELINs from the first group could equate to floor 1, Pillar 6 at a company’s address and ELINs from the second group could equate to Floor 2, Pillar 3 at that same address.

ERLs can also be associated with wireless adapters for in-building mobility.

I should point out that there are a variety of companies (911 ETC, Conveyant, RedSky, 911 Enable, Amcom, etc.) that offer products that work with your PBX to assist in mapping IP telephones to ERLs.

ERLs and ELIN mapping doesn’t solve everything, though. You need to make sure that the ELIN is sent to the correct place in order for the Selective Router to find the appropriate PSAP. This requires 9-1-1 calls to be sent out on the right trunks.

9-1-1 routing isn’t overly complicated when trunks coexist with the users they serve. This means trunks exist at both the main site and any gateways off the PBX. Route tables are configured to send 9-1-1 calls out the appropriate local trunk no matter where the telephone is receiving its call control processing (main or gateway).

It becomes complicated for branch offices when a company eliminates trunks from their remote gateways and centralizes all calls on SIP trunks. In this case, there is no connection between a remote location and its PSAP.

Thankfully, there are companies that offer cloud based solutions to map remote IP users to the correct PSAP. Three that come to mind are RedSky, 9-1-1 Enable, and 911 ETC. All allow you to send 9-1-1 calls directly to their clouds. Their clouds will then route 9-1-1 calls to the appropriate places. For example, you might consolidate all your trunks in Chicago and have a “trunkless” gateway in Minneapolis. If a Minneapolis user makes a 9-1-1 call it will be sent out a SIP trunk in Chicago to a cloud service which will then send the call back to Minneapolis and the correct PSAP.

Mobile Users

Unfortunately, there is still a hole when it comes to mobile SIP clients. I could be wrong, but I don’t know of any solution that allows you to place a 9-1-1 call from a SIP client on an iPhone or Android connected to a 3G or 4G network. In this case, the call must either be disallowed by the client software or the soft-phone must place the call on the cellular connection.

At some point, I would love to see SIP messages imbed location information such as GPS coordinates and have that used to route calls, but I know of nothing on the horizon that allows for that.

Wrapping Things Up

I didn’t expect this article to be this long, but once I got going it was hard to know when to stop. Hopefully you’ve learned something over these past three or four pages and are now prepared to dig into 9-1-1 even deeper than I’ve taken you. If you are going to implement 9-1-1 on your PBX, I would highly recommend that you take the extra steps to become even more knowledgeable. You don’t want to mess around when it comes to life or death situations.

For a different look at this interesting subject, I suggest that you read my article, Next Generation 9-1-1.



  1. fletch911 · · Reply

    Excellent coverage Andrew, and it’s great to see others covering this important topic! While complex networks need specific functionality, it is also important to remember that even the simplest environments can take steps to ensure basic functionality is operational and correct.
    This is the entire premise behind Kari’s Law and the initiative targeting hotels, motels as well as businesses that operate PBX systems.

  2. Thanks, Mark! I always welcome your input on this important subject!

  3. Thanks Andrew for covering this topic and the mention for Conveyant Systems!

    1. You’re welcome, Tim! Thanks for reading and taking the time to comment.

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