Say I have a PBX system and want to connect the outside line to another PBX unit (as an extension). And say I want to connect that to a larger PBX, etc...

So, assuming that 9 is the outside line selector at each level, it would look something like when you dial the number 555-1212:

Not on PBX: 555-1212

Top Level PBX: 9-555-1212

1st Nested PBX: 99-555-1212

2nd Nested PBX: 999-555-1212

3rd Nested PBX: 9999-555-1212

...and so on...

(the PSTN is hierarchical in structure so this doesn't seem that radical)

Would there be any complications in such a setup, where you just move up the stack at each level?

Note I am not asking about a VoIP or digital setup: just a regular analog system and analog phone. As such, there would no concept of a "dial plan" in this system. There is literally nothing like to configure.

Here is a forum that exemplifies exactly what I'm talking about: http://www.classicrotaryphones.com/forum/index.php?topic=16612.0

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 17, 2017 at 4:23
  • @RonMaupin Your answer was helpful but didn't exactly answer my question. I was asking specifically about analog PBXs with analog trunk lines, so there would be no FXS/FXO ports. Or SIP. Or PRI. Caller ID after the first ring, although I don't care much for Caller ID since I don't have any phones with a display anyways. Commented Aug 17, 2017 at 9:56
  • I did not provide an answer to this question. I was simply asking if one of the answers helped you, or if you had your own answer. We have a bunch of questions that have no accepted answers, and I am trying to get that cleaned up. So far, I think there have been over a hundred that have accepted answers based on my inquiry.
    – Ron Maupin
    Commented Aug 17, 2017 at 14:50
  • @RonMaupin I don't know if it's best practice to make the best answer as accepted, or simply not accept any answers until you are satisfied with one. Right now, I am not satisfied with any because I asked about analog and both answers were about digital or VoIP. If you want me to just mark the better one, I can do that. Commented Aug 17, 2017 at 19:43
  • That's fine. I'm just seeing if we can clean up any that may have been forgotten. If you don't have an acceptable answer, then you don't need to accept an answer.
    – Ron Maupin
    Commented Aug 17, 2017 at 19:45

2 Answers 2


Yes, that is possible - the local telephone exchange is basically just a large PBX (without the P/Private bit).

Depending on your PABX, you may not even require multiple layers of trunk access code to get an outside line, but that would be device-specific.

  • PABX? What does the A stand for? And I am talking about a completely old school analog setup. So would this work for as many layers as needed? Meaning theoretically, I could have a setup where you have to dial 99999999999990 just to reach the local operator? Commented Mar 12, 2017 at 22:44
  • A - Automatic - used interchangeably with PBX. It would depend on your system, but in theory yes. There would have to be an upper limit on digits dialled somewhere, and you upstream PBXs would need to allow for each extra digit as well. Commented Mar 13, 2017 at 8:41
  • Why would there have to be an upper limit? How is each PBX even aware of the ones downstream from it? If we use Strowger switches as our physical equipment for example, a Strowger switch has no idea how many switches a call has gone through already: so isn't the number of nester PBXs technically infinite? Commented Mar 13, 2017 at 9:32
  • The PBX your handset is connected to needs to be aware of the dial-string you are entering - e.g.: the difference between 9998 (possibly an extension) and 9999999 (your hypothetical stacked PBX outside line number), so yes, there will have to be some limit placed on the number of PBXs you pass through, as you will need to define the length of recognised dial-strings at some point. This is all very hypothetical though - why would you want to do this? Commented Mar 13, 2017 at 10:19
  • Read it again: "The PBX your handset is connected to" Commented Mar 13, 2017 at 12:29

Nesting is not a commonly used term in telephony. Extension numbers, aka directory numbers, or direct inward dial numbers, are defined in what is called the dial plan.

In terms of routing calls towards the Public Switched Telephone Network, PSTN, this is sometimes called route pattern or destination pattern matching. Prefixing outside calls with a 9 is not necessary but is common.

A PSTN analog line is an FXO port. While is it possible to connect FXS to FXO ports in-house, you practically never see this. Usuallu\y, a PBX is going to have PSTN connection(s) directly into it, preferable digital (ISDN PRI) or SIP trunks so you can get calling name and number and DID functionality.

  • If you read my question, I am referring only to all analog systems. I know this is not common, I just want to know if it is possible Commented Mar 14, 2017 at 10:00
  • The term key system may be applicable (vs PBX)? In any case, yes, you can daisy chain them off one another relative to the outside line / 1FB / dial tone / PSTN. Commented Mar 15, 2017 at 3:28
  • Key systems are manual. This would all be automatic. So I believe I am referring to a PBX, one without a dial plan or any configurations. Commented Mar 15, 2017 at 8:55

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