PBX is designed for use by single organization or an office of single organization (although there ways to share PBX among multiple offices for economical reasons). Main benefit of PBX its ability to create internal, company owned telephone system. PBX circumvents the need to purchase dedicated telephone line for each employee that needs to have the telephone. In the typical mid-large size business other than Call Center or Telemarketing Company, each employee on average is making very few calls and therefore few telephone lines can be easily shared among larger number of users. For example a typical office of 100 employees may safely share 20 external lines as external line is only used when user is making the call outside of the office.  Typically there is no limitation on number of simultaneous phone calls between phones connected to the same PBX.
Second common benefit of PBX is ability to use single telephone to reach the company from outside world. This concept makes it much easier for any external party to reach your company via single number and reach appropriate department or individual by navigating greeting menu or entering extension of the user. Ability to advertize a single phone number is just one example why PBX is widely used by majority of businesses. List of additional features and benefits is endless and usefulness of them depends on type and size of business.
PBX is typically terminating less than 100-200 external phone numbers and for small and/or home operated business sometimes as little as one or two phone lines. Connections between PBX and outside Telecom Company are called “Trunks”.  
   Internal phones are called “extensions” and phone calls placed between those extensions are free meaning that the company that owns/operates PBX does not have to pay for calls that are placed between internal extensions.  When internal extension is placing the call outside of the internal PBX serviced space (office), Telecommunication Company (TelCo) is charging the fee for use of their infrastructure. On the contrary, when call is placed from one internal extension to another internal extension the call is free of charges by TelCo. 
    Depending on the type of PBX, external lines (Trunks) can be of a different kind. The most basic type of external telephone connection is an analog line. This type of line is also known as POTS which stands for “Plain Old Telephone Service”. The next step from analog line is a digital line called T1. Single T1 can simultaneously service 23 (PRI) to 24 (CAS) independent phone lines depending on type. PRI (23 lines) is more widely adapted as it offers more call features than CAS T1.  T1 can service less than 23 lines though and becomes cost effective compared to analog lines when as little as 6-7 lines are purchased from Telephone Company (TelCo). 
    The newest and most inexpensive type of external connection is delivered via VoIP (Voice over Internet Protocol). The biggest benefit if such solution when it comes to adapting VoIP to interface with PBX is its cost and flexibility.  Businesses have much wider range of providers to choose from. Vast majority of Internet Telephone Service Providers (ITSP) will assign you the number from area code your business resides in, or any other area code in the country or combination of both. Business can purchase VoIP Trunk that can service as little as one simultaneous external call or any number of calls deemed appropriate for your business based on peak volume of simultaneous external calls. 
   For example your business is located in Philadelphia and majority of your customers reside in Philadelphia, but significant portion of them reside in New York. ITSP of your choice will allow you to purchase two numbers; one with Philadelphia area code of 215 and second number with area code of New York.

As the result:
1. Calls between your office and customers in NY are free of long distance charges.
2. Customers in NY are receiving calls from number that is local to them.
3. If your office moves to NY (or any other location in the world), you get to keep your Philadelphia and New York numbers as well as can get more local phone numbers in the area you have moved to. 

To be continued…