Direct Dial In (DDI) numbers are special numbers that let your telephone extensions have an individual number. They work on your main ISDN network. Not all equipment support DDI but those that do, will allow a call to a specific extension directly. DDI numbers work well with ISDN lines that require point to point configuration. This is alternatively called T reference configured lines or a system access.
When a user wants the DDI number presented on outgoing CLI, make sure the PBX supports this feature. Usually it is the main line number or the directory number that shows on CLI. These are the main numbers that are billed by the business telephone service providers. You will have to configure your PBX to override this and do call billing for the DDI number.
In contrast Multiple Subscriber Numbers (MSNs) are numbers that can be additionally associated with your ISDN circuit. They let different numbers to be allocated to telephones, faxes and computers. Make sure your devices are programmed correctly so that the right device responds to the allocated MSN.
MSN work with ISDN lines that operate with point to multi point configurations. This is alternatively known as standard access or S/T reference point configured lines. This configuration will work well with ISDN terminals, PC cards and your telephone devices. But for PBX users, it will be better to use Direct Dial In numbers.
You can purchase a DDI number in blocks of 10 or as a single DDI number from ISDN30/30e Business sales. Usually there will be 6 digits that are sent down the ISDN line to your equipment. You can contact PSTC to get it changed from six digits to four digits; given you are ISDN30e (ETSI) customer. If you are an ISDN30 (DASS) customer, you will have to contact ISDN Business Sales.
The corresponding service to DID is called direct outward dialling (DOD). It is alternatively also called direct dial central office. Often both services are provided in combination. This will allow dialling in and dialling out, on and to Global telephone numbers from extensions covered by these services. This means that these extensions will not need operator service.