All you want to know about DID !
Here is a short example to explain about DID from your daily life. You might have made a call at an office reception to speak to someone in the office. The receptionist would have kept you waiting and transferred your call to the concerned person. This is because the receptionist has transferred the call through PBX system and DID have not been used in this scenario. If you would have the direct number of the concerned person with you, maybe from his personal business card, you would not have to call at the reception and then transfer the call. This way you could get directly in touch with the person by dialing his direct number. This service is called DID where the company has given individual telephone number connected through PBX.
You must have seen that companies have series of phone numbers for their customer service. What happens is, a company might rent, say, 100 telephone numbers from their local phone company that could be called over ten physical telephone lines. These ten physical lines are called "trunk lines". This would allow up to ten ongoing calls at the same time. If there are any additional inbound calls coming, the caller would get a busy tone until one of the calls is completed or the caller will be channeled to a voice mail message. Under this service, the PBX automatically switches a call for a given phone number to the appropriate workstation in the company. However, a PBX switchboard operator is not involved.
By using this feature one can one can split up their telephony traffic and manage it more easily. A DID system does not require a PBX operator and can be used for fax and voice transmissions. A DID system can be used for voice mail, fax and for live voice connections. As Compared to the regular PBX service, DID service saves the cost of a switchboard operator, it makes the calls go through faster, and the best feature if DID service is that the callers feel they are calling a person rather than a company.