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.