Many companies already have a phone system (PBX) where each extension can be dialled directly from outside the company, rather than having to go via the switchboard. The technology behind this is called Direct Dialling Inwards (DDI or DID in the USA), and is as applicable to fax, data and video as it is to voice calls.
This is the most direct of the inward routing methods. Each user is given their own telephone and /or fax number - not a separate line, just their own number. Voice calls, data calls, video and faxes calls sent to that number are routed directly to the user. A difficult concept for some people to understand is that ISDN allows infinite telephone numbers whilst only renting a small quantity of channels (lines) for example it is possible to have say two telephone lines (channels) (i.e. Two calls can be made at one time without being engaged, voice & fax, voice & voice or voice and data) but 100 telephone numbers. In other words any two of the numbers (or one number twice) can be used simultaneously.
DDI means that the number dialled is transmitted as well as the number of the calling party. This has powerful marketing analysis capabilities. For example ISDN users who have enabled DDI will have been allocated a DDI number range of at least 100 numbers by BT. Some of which the company will allocate to their main telephone numbers and fax machines. Others however can be used for example when placing advertisements, using a different DDI number for each advert is an easy way to monitor exactly which ads are producing the best response. If you have call management software the data is captured to a computer system for even greater analysis.
You can also use DDI numbers to remove the need for extra lines just so that you know to answer in a different company or department name. If you need to answer different DDI numbers with a different greeting then your system must support DDI tagging. But beware, when you activate DDI tagging on some systems you lose the ability to see CLI.