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I'm trying to understand how to properly configure a PBX (Private Branch Exchange). I've stumbled into a few brick walls, and I feel like the answers to these should be obvious, but the Internet has not helped at all. After consulting stackexchange.com/sites, this seemed like the best place to post regarding telephony networking,

First of all, does a PBX require its own power supply? Standalone telephones work in a power outage since they are directly connected to the central office, but in a PBX setup, the telephones are connected to the PBX, which is linked via outside lines to the central office. So, would the PBX/telephones work in an outage, or does it need backup power? If your PBX is Centrex (hosted at the exchange), will the telephone company supply power to your PBX in an outage?

How does one actually configure extensions in a PBX? How would one set the system up so if you dial '1010' it rings a specific line somewhere in the building? Since there is no software or computer-interfaces in a PBX, and it is analog, rather than digital, how does the system know which line is which?

Also, does each outside line need to be paid for individually, or are there trunk leases? If everyone dialed out at once, would some users get "no circuits are available" if you have 50 internal lines but only 25 outside lines? Would you need as many outside lines as inside lines? And I know many organizations have assigned prefixes (they own all the numbers in 555-345-XXXX) where they own all the X numbers. Do you just ask the telephone company to lease an entire prefix, or part of a prefix?

Also, if the PBX loses power, can I just unplug an outside line from the PBX and plug a telephone into it? Is there any reason that wouldn't work? (like if the line is part of a trunk line instead of an individual cable)

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First of all, does a PBX require its own power supply?

A PBX does require it's own power, and you must have batteries or a UPS in the event of a power outage.

If your PBX is Centrex (hosted at the exchange), will the telephone company supply power to your PBX in an outage?

Most telcos will insist on and install a UPS for any of their equipment which they install on your premises.

How does one actually configure extensions in a PBX?

How a PBX is configured depends on the particular make/model. It can be pretty complex on some systems, and most PBX vendors offer training (for a price).

Since there is no software or computer-interfaces in a PBX and it is analog, rather than digital, how does the system know which line is which?

Many PBXes use software and have computers and network interfaces built in. Also, there are both analog and digital PBXes, as well as VoIP PBXes. Analog and proprietary digital PBXes are circuit switched, so the PBX know which phone is which by the circuit to which it is connected, they same way the telco knows which line is yours.

Also, does each outside line need to be paid for individually or are there trunk leases?

How circuits are delivered depends on the PBX make/model, and what your local telco offers.

If everyone dialed out at once, would some users get "no circuits are available" if you have 50 internal lines but only 25 outside lines?

If you have more users trying to make outside calls than there are outside lines, then you will get some sort of message or tone indicating a problem when trying to dial an outside line.

Would you need as many outside lines as inside lines?

There are various calculations (Erlang), depending on the business model, to determine the number of outside lines you should have based on the number of users. This is almost never close to one-to-one, but there are exceptions, depending on the type of business.

Do you just ask the telephone company to lease an entire prefix, or part of a prefix?

Most telcos will assign/sell you one or more DID (Direct Inward Dial) range of numbers, and these can be much larger than the actual number of outside lines that you have. This can be an entire exchange (expensive), or a small range (e.g. 10 numbers).


Seriously, and I mean this with respect, if you need to ask these questions, you should not be doing this; you should hire a professional to do this for you. This is very easy to get wrong, and your business is on the line (no pun intended).

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  • Wow, I am surprised at the details - so to clarify, to guarantee uptime on all lines, I would want as many outside lines as inside lines? I was planning on a completely analog PBX with legacy rotary switching equipment (I will need to support pulse dialing from rotary/switch-hook). Luckily, this is not for work - I was thinking of implementing this in my household someday since I will have a line (or multiple lines) in each room and I want free internal dialing, etc... – InterLinked Jul 15 '16 at 18:33
  • If you have the same number of outside lines as inside lines, you are typically wasting money. For Internet use, you need a VoIP PBX, and you will probably buy the service from an Internet telco. Unfortunately, questions about home networking are explicitly off-topic here, but are supported on Super User. – Ron Maupin Jul 15 '16 at 18:37
  • I was talking about a real analog PBX, not VoIP. And this isn't specific to home networking - it will be a fully commercial PBX with a commercial PBX setup so it makes no sense to call it home networking. Not many homes out there with a PBX and 25 telephones – InterLinked Jul 15 '16 at 19:13
  • A regular PBX doesn't use the Internet for calls. Calls on the Internet are VoIP, and that means it would make sense to use a VoIP PBX. If you want a DID block of numbers, you will need to use a telco service, whether it is analog, or VoIP with a SIP trunk (direct from a local telco, or over the Internet). – Ron Maupin Jul 15 '16 at 19:49
  • I know what VoIP is - I'm saying I would not consider it or anything else besides true copper-wire analog infrastructure. Do the outside lines just connect to one side and the inside lines to the other? – InterLinked Jul 16 '16 at 1:52
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A PBX is typically implemented in organizations that have many users who each require a telephone. They typically dial one another via 4 digit dialing.

If a PBX phone user wants to call and order a pizza, they typically dial 9, for example, to obtain an outside line, then dial the public (PSTN) phone number of the pizza place. A single phone line is required for every concurrent call from the PBX system to the PSTN.

So, organizations can get analog lines for PSTN access or they can get digital lines if they have enough users/need. Nowadays SIP is another PSTN offering.

  • Analog Lines: Incomings Calls Must All Ring The Same Number, whether assigned to a single phone or multiple phones. Calling Number can display on PBX handsets after the 1st ring.

  • PRI: Incoming calls can ring a particular number. This is called DID. Calling name and number can display on the PBX. This is the bonafide classic PSTN trunk type. You must get one delivered to your building and you get local numbers with it. 23 calls concurrent max. Faxing works fine.

  • CAS T1: Another type of trunk but with less features than a PRI. 24 concurrent calls max.

  • SIP: An IP (vs 'digital') trunk whereby you can get numbers from area codes all over the country ringing in. Faxing across these can be problematic.

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  • I believe what I want is PRI - so I can't have more than 23 outside lines (or is that internal lines?) Is it possible to configure a PBX so that you don't require 9 to obtain an outside line? Can it just 'know' if it isn't a 4-digit number that it as an outside number? Do vertical service codes work with 9? I tried using *67 on a phone where 9 was required to obtain an outside line and it didn't work? And to support rotary and pulse dialing within the building, do I need electromechanical rotary switching equipment on premises? – InterLinked Jul 16 '16 at 12:02
  • Whether PRI, Analog Line(s), or SIP - the connection between the PBX and the PSTN is typically called a trunk. The trunk is only invoked/used for calls between the PBX endpoints/phones and public/PSTN numbers (or phones). Calls between PBX phones do not traverse the trunk. In terms of 4-digit vs 7-digit and dial 9 for outside line or not, that is all referred to as the dial plan. The Dial plan is entirely configurable on the PBX. It is not necessary for a PBX's dial plan to have users press 9 for an outside line. – Ronnie Royston Jul 16 '16 at 16:52
  • Then what is the advantage of requiring users to press 9 for an outside line? And will vertical service codes and pulse dialing both work? What if I need more than 23 concurrent calls? – InterLinked Jul 16 '16 at 20:42
  • It simplifies the dial plan of very large organizations. Yes service codes will/can still work. Pulse dialing is specific to analog telephony ports (analog lines). – Ronnie Royston Jul 16 '16 at 21:49
  • If I want to use pulse dialing internally, do I need a Strowger switch (or multiple) internally connected to the PBX? Will external pulse dialing also work? And what about 23+ concurrent calls? – InterLinked Jul 17 '16 at 0:21

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