Ask Mr. Andrews: What is a DID?

Q: Dear Mr. Andrews, What is a DID?

A: The term “DID” stands for “Direct Inward Dial”, and describes a feature offered by traditional telephone operators. For example, my direct number at work “716-250-3402” is a DID number, which we essentially “rent’ from our provider. Telephony companies allocate a range of DID numbers within each local exchange and area code, and dole them out to their customers in much the same manner as one might be assigned an email address or a website domain. (i.e DID numbers are “unique”)

The technology behind “DID” was originally developed in the 1960’s by AT&T, with the intent of allowing companies to have fewer lines than extensions, while still maintaining a unique number for each internal extension, which can be called from outside the company. So DID (“direct inward dialing”) was invented as a way to re-use a limited number of physical phone lines to handle calls to different published numbers. In a business with DID, the phone company uses DID signaling to identify the number they are about to connect to the business’s PBX. Historically, this was done by pulsing the last 3 or 4 digits of the number being dialed before connecting the number. The PBX would use these DID digits to switch the call to the right recipient.

DID numbers are typically associated with business use through a corporate PBX (Private Branch Exchange), but also have applications within the residential market. Each extension within a PBX system may be assigned a DID number. An external caller may bypass the operator or IVR/Auto-attendant of the PBX by dialing the DID number or the extension they wish to reach. In my case, my internal extension is 3402, and my DID number is 716-250-3402. People who call me can dial our main 800 Toll Free # and then enter my extension, or alternatively can dial my DID number directly.

DID numbers have particular importance for VoIP communications. In order for peple connected to the traditional PSTN to call people connected to VoIP networks, DID numbers from the PSTN network are obtained by the administrators of the VoIP network, and assigned to a gateway in the VoIP network. The gateway will then route calls incoming from the PSTN across the IP network to the appropriate VoIP user. Similarly, calls originating in the VoIP network will appear to users on the PSTN as originating from one of the assigned DID numbers.

Let’s say you buy a phone line from Vonage or some other phone service provider who offers phone service over broadband. The number that they provide to you, in technical terms is a DID number. This is the number that they have assigned to you to connect you to the old PSTN Networks around the world. Any service provider who wants to offer a phone service over IP address, needs to buy DID numbers from his CLEC or any other large service provider like Level 3 in the United States or go to a consortium (company that will take large blocks from many providers and hand them out one at a time)

If you are using an IP PBX like Asterisk, and you want to connect yourself to PSTN so people can call your office, you can either:

buy an Analog or E1/T1 card from Digium, Sangoma or Rhino.
buy DID number from a DID service provider.

DID Service Providers, convert the analog to digital and provide these DID numbers over the internet, with SIP or IAX2. Service providers like DIDX and Voicepulse do this. You buy the number, and send it straight to your sip address, and you are good to go. The call will then come to your IPPBX as a real phone line. Then you can use as your phone number, and route it to your IVR or direct extension.

Garrett Smith

Garrett is the former VoIP Supply CMO.

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View Comments

Tom Patton says:

August 27, 2008 at 2:40 PM

I would like to do origination from Puerto Rico and would like to access an inventory of DID's.. I have been to http://www.didx.com and can get some there however they are expensive and they only have a limited number available..

Thanks!!
Jorge Arevalo says:

August 27, 2008 at 3:03 PM

What's a CLEC? Is is the Carrier? About buying DIDs. Who are some known corsortiums that sells DIDs? Are there any that sells DIDs in different countries? So I can buy DIDs in the US and Mexico for example and put them in my Asterisk...

Thanks! Great Article!
Cory Andrews says:

August 27, 2008 at 7:58 PM

Jorge - CLEC stands for "Competitive Local Exchange Carrier". A CLEC is a company which provides local telephone service, an alternativeto the the incumbent Local Exchange Carrier (LEC).

In the same manner as a LEC, a CLEC provides service within a LATA. A LATA (Local Access and Transport Area) is a geographical and administrative area that is the responsibility of an LEC (Local Exchange Carrier).

To handle InterLATA long-distance calls, the CLEC will connect to an IXC (IntereXchange Carrier).

Most CLEC's will use the local loop (UNE-L) belonging to the incumbent Local Exchange Carrier.

The Telecommunications Act of 1996 forced the incumbent LEC's to lease access to their local loop (UNE-L) to CLEC's at wholesale prices. Between 1996 and 2004, the FCC also required LEC's to lease other network elements (UNE-P) to the CLEC's.
Cory Andrews says:

August 27, 2008 at 8:04 PM

Tim and Jorge - As Tim mentioned, DIDx.com is one of the more prominent providers of DID numbers. While not least expensive provider out there, they do provide excellent service, and are a reputable company to be working with. If you are only looking for a handful of DID numbers, sometimes its worth paying a bit more and going with a well established provider.

Some other companies that provide DID numbers include Voxbone, Mydivert.com, NXVoice.com, DIDSite.com and many others.

You might also consider going to http://lists.digium.com and subscribing to the Asterisk-Biz listserv. Once you have subscribed, send a message to the listserv community stating that you are looking to obtain international DID numbers and you will likely be contacted by quite a few providers.
Adi says:

February 5, 2009 at 3:58 AM

Hi,

I found this website by searching for reviews of DIDs providers. As we know there are many of them in the market but not all of them are good enough. I think I found something I was looking for. I needed more info about Mydivert.com I`m thinking to purchase DID number for use as CallIn only, rather than CallOut. They seems to have the best price on the market, but I don@t know them and never heard of. Could you please confirm with me your post from 2008 that they are still in business and are fine. Thank You
trevor york says:

March 17, 2009 at 4:21 PM

i saw your article and thought it was informative . here is what i am trying to a wholesale distributer of did numers where i hear they can be purchased from .01 to 1.00 a piece, i also need a carrier that will load number in 411 directories across the country and some internet directories. i had a company that was selling 1.50 did per month and 1.50 listing per month but wanted 5.00 to set up a did and i need 2500 did nos. i figured out that he was buying from someone probably a clec that load directly. if i can get a company that will load in the directories ,i have found a price as cheap as 1 per did. can you help i know this is a lot of info
mydivert virtual numbers says:

May 25, 2010 at 6:55 AM

Hi Adi

Thank you for your interest in DID numbers.
Yes we are still in business and if you are looking for any help or information please take a look at our website or contact us at sales@mydivert.com

Best Regards
Stephen Bray
David Braatslund says:

October 1, 2010 at 11:08 AM

Can you address this issue which perplexes many of us? When we buy a traditional PBX you can get blocks of DIDs for under a dollar a DID. When we try to migrate to a Hosted VoIP solution we are often told that they provide a main number and additional DIDs are priced at a mark-up, like $5 per DID. I know historically some Hosted solutions had problems providing unified Caller ID, so they had to give a direct dial number for each extension. More advanced companies, who could handle that, would not, claiming that most companies don't need the extra numbers, and it costs them like $3-$4 a number to get them from companies like Level 3, so if you want extra DIDs you have to pay for them.

I recognize that some clients need DIDs in other areas, to look local in multiple cities, and $5 per number per DID is a real bargain, over having to order a virtual number from a LEC, multiple line paths, and Local/Long Distance forwarding charges. This can't be beat with traditional service providers, which charge like $20 for the virtual number, extra for more line paths, plus the substantial forwarding charges

For companies that just want to keep a large block of numbers in their local exchange, why do hosted provider's DIDs cost so much more than DID blocks associated with customer managed PBXs? I have heard stuff regarding session charges and origination fees and advanced features being transmitted, etc. Still very confused.