Direct Inward Dialing Number (Also known as DID or DDI)
DID (DDI) Background
Most businesses have several incoming telephone numbers used for specific purposes. For example customer service, sales, etc. Some have an individual telephone number for each user in the system. In a home setting on the other hand, each telephone number comes in on a different pair of wires typically. This is not practical in a business enviroment that has many telephone numbers.
DID (DDI) in the new VOIP World
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 1) buy an Analog or E1/T1 card from Digium, or 2) buy DID number from a service provider like virtualphoneline.com, broadvoice.com or voicepulse.com that 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. You can get a FREE UK Did from IPStar.us or buy commercial DIDs from service providers like virtualphoneline.com