The modern enterprise have DID which is stands for Direct Inward Dialing so this option is as a service from their telephony service provider. This is not a new concept or thing which has been here for a long time. Before supporting the technology delivering this service has evolved so, the basic concept remains the same as before.
The basic thing about telephony is that it is dedicated resource. Which means that every customer number would be assigned a dedicated resource from end to ends, so even any subscriber line had an associated cable with it. This is good for any customer who have single line. But for enterprise customers with many employees with individual subscriber lines were expensive than other one.
So with this DIDs (direct inward dialing) there is the problem which were resolved that this allowed the enterprises to install a hosted PBX that could help this service and a single trunk from the CO to that hosted PBX. This CO switch allocates a large numbers of the trunk so, one of these numbers were dialed the CO would send it over the trunk and PBX will forward the call to the right extension.
This thing was a best solution for the organization as well as for the service providers, for the enterprise, this mean the less operators have to handle the calls but on the other side the cost of DID service was lot of less as compared to individual telephone lines. For the service provider have same structure that could be used for multiple telephone numbers which is easy for communication and less costly.
When telephony and communication networks have involved into ISDN networks so, the DID (direct inward dialing) have the circuits to move from an analog trunks to ISDN based solution. Usually, these numbers are to be used over an ISDN trunk. This trunk would connects with the hosted PBX and to the CO switch just like the analog trunks, but it was digitized. This solution is mostly common like everyone is using this software it is too much famous in the market now.
The last thing which you have to keep in your mind that here, are some of the import things of this concept in Voice over internet protocol networks too. Many people would not believe that the DID would be relevant in Voice over internet protocol things. But it can be used to allocate the number block to the communication gateway. When new Voice over internet protocol phones are connected and configured with this, the gateway numbers are assigns to the respective handsets for the communication, this software is too much easy in the use and this software also increase the productivity of the organization.