A PBX is used to efficiently share resources of many different phone numbers and many phone lines within a business. A PBX is typically used in larger environments where employees are located in separate offices. Most telephones in the PBX system have a single, unique per-phone extension number. A receptionist at an attendant console or an automated-attendant receives all incoming calls, which are transferred to the requested extension number.
A PBX will usually connect to the PSTN with ISDN PRI line and DID numbers. An ISDN PRI is essentially one line that is capable of carrying up to 23 phone calls at one time. DID is an acronym for direct-inward-dialing. DID is a service offered by the telephone company.
DID numbers are purchased in a block of phone numbers and associated with one or more phone lines. DIDs are used to assign a dedicated phone number to each employee, without requiring the purchase of a separate phone line for each employee. In doing this, telephone traffic can be split up and managed more effectively.