A PBX, or Private Branch Exchange, is a term that comes from the days when phone systems were just starting to come into existence. A PBX was a phone system that allowed a way for telephone lines to be shared by more than one person. Before a PBX, everyone needed their own direct connection to the CO, or Central Office, of the service provider. This got expensive and complicated, and businesses quickly identified a need for a more efficient way to operate.
Enter the KeySystem – a PBX which takes incoming lines from the CO and basically duplicates them across all the phones in the office. You can actually still find these in use today – or at least the functionality. If you've ever walked into an office that has several lights on each phone, and they all blink and ring together at the same time, that was likely a keysystem.
A more traditional PBX came after that – allowing the users of each phone to have their own internal extension. They would then share an outside line, usually by dialing ‘9' – but in the old days you used to call an operator that helped run your phone system to connect you manually. You can still find this “dial 9 to get an outside line” in use today – which is sadly no longer a technical requirement and instead exists to appease slow adoption. People don't seem to like change, sadly.
Just kidding. Well – kind of. A common requirement for getting rid of the “dial 9” thing is to give everyone a full 10 digit phone number. Since people don't like to remember 10 digits to call their co-worker, people seem to cling to the 4 digit extension era. As technology advances though, and people more commonly “click” to dial, by selecting a contact from your phone for example, and no longer need to know phone numbers – we'll see “dial 9” start to fade away.
People today still tend to refer to all phone systems as PBX's. Though, in reality, they're more closely akin to a Telephony “Server” or “Communications Server”. Common suppliers like Cisco and Microsoft call their products “Unified Communications Systems” as they tend to encompass telephone, instant messaging, and video all under one system. Given enough time, we'll start to see PBX's used less and less.
If you want to know how modern phone systems work,
I invite you to take my Udemy Course: Introduction to SIP! Use the coupon code TA-Youtube for 50% off. Click here to go there now.