Analog Trunk between TWO PABX System

[harmitin]

Hi, We have a strange requirement from our customer to connect two PABX systems over an analog trunk.

Currently, analog phone is connected to PABX A over 1 pair cable. 

Our customer wanted to connect PABX B in place of the Analog phone to have a one-line connection between both PABXs. The purpose of this connection is to allow only calls between PABX users when an access code is dialed.

Could you please help me, what configuration should I proceed to make this interface working?

Client dont want to use SIP trunk to inline with network segregation requirement.

[Elliot Dierksen]

This type of connection should work if the connection on one side is FXS (where you would plug in an analog station/phone) and the other side is FXO (where you would plug in an analog phone).

The next question is what traffic do you want to route over it, and how? How meaning are you expecting to send digits to the other side, or do you want an off-hook condition to route the call to a statically defined destination? Sending digits out an FXO port is likely not a problem. Whether the PABX in question would allow you to send digits out the FXS port would be a question.

[harmitin]

Thanks.

so PABX A - FXS and PABX B will be FXO port.

the purpose is, when someone dial from Pabx B to pabx a, it should be possible via access code followed by number. From pabx A to pabx b, not sure how call willl be interpreted. Please suggest 

[Elliot Dierksen · Accepted solution]

My area of expertise is Cisco VoIP, not PABX systems so I don't know how much help I can provide there. That said, you could have an access code/destination that points to these ports on either side. That should ring and connect. Then the user could enter the subsequent digits to connect. That would mean a two step process for the users. Whether the PABX could collect <access code><destination> and then only forward <destination> after the connect will be PABX specific. I can't help you there, even if you tell me who manufactures them.

[harmitin]

I think this is enough for me to on the right oath. Thanks for your help. 

[Jerrytech1]

This is a basic Tie Line. When they dial the access code, they receive dial tone from the other system and dial extensions there. If they want it to work in both directions, you will need 2 tie lines. From A>B and B>A

[harmitin]

Hi, Thanks for your reply.

You mean...If PABX B wanted to make calls to PABX A, they would dial the access code and the destination number. 
PABX A (FXS) and PABX B (FXO).

can we make calls from PABX A to PABX B? Please guide me.

[Jerrytech1]

Thats correct. If you want two way calling, you need this set up on both sides.
PABX A (FXS) and PABX B (FXO)
PABX B (FXS) and PABX A (FXO)

[Roger Kallberg]

An opinion if possible is to use an E&M port that supports duplex direction. Besides that it’s possible to both receive and send calls on a FXO port as that’s what you’d do in a voice gateway if you use analog connection(s) to/from PSTN.

[harmitin]

Thanks. Why FXO port is acting different when we connect it via gateway to pstn in comparison to connection with fxs port? My understanding was, pstn and pabx both provides fxs ports so call should work in both way on a single line.

please guide.

[Roger Kallberg]

That would be my understanding as well. I don’t think that it’s much of a difference.

[Elliot Dierksen]

I hesitate to go to far in this because my knowledge is more specific to Cisco voice gateways. There may very well be restrictions in the PABX on what you can do with a particular kind of port that don't exist in a voice gateway. Think of it this way. An FXS port is where the PABX expects a station to be, something like an analog phone or a fax machine. Generally speaking, these are things that you wouldn't expect to being sending additional digits. An FXO port is where you plug in an analog line. You clearly have to send additional digits to that to get it to route calls. A Cisco voice gateway would let you configure a dial peer that forwards digits to an FXS port. I know this because I have done it myself numerous times.

Another possibility would be to use an intermediate device like a Cisco voice gateway. You could have one with 2 FXS ports. 1 port would connect to an FXO in PABX 1, and the other to an FXO port in PABX 2. That would get you around the PABX limitation that only lets you forward digits to an FXO port. I'll use an example of a 3 digit dial plan. We assign a prefix of "1" to 3 digit extensions in PABX 1, and a prefix of (wait for it) "2" to PABX 2. The gateway would be configured to send destination 1... (1 plus any 3 subsequent digits) to the port (or ports) attached to PABX 1. A similar dial peer would route 2... to the PABX 2 port (or ports).

Hopefully that makes it clearer. @Roger Kallberg is correct about E&M analog port being capable of bi-directional function, but those may be harder to find these days. I can't recall the last time I used an E&M card in the wild.

[Jerrytech1]

I am trying to keep it as simple as possible as well. It's a straight forward tie line. Easiest solution is loop start lines on both sides with a secondary trunk access code. Dial the code, get dial tone from the other PBX, dial the extension.

[harmitin]

Thanks for the out-of-the-box solution.

@Roger Kallberg   @Jerrytech1  @Elliot Dierksen 

I got the clarification from our client that from the FXS (PABX A) to FXO (PABX B) side, they want a call on the predefined extension on PABX B from PABX A, so there is no need to send the additional digits from FXS to find the extension number in PABX B as this will be predefined and always will be the same. From PABX B to PABX A, we can make calls to any number as we can send access code via FXO port. 

Could you please suggest whether this will be possible?

[Roger Kallberg]

Any thing, or most at least, is possible. It’s just a matter of configuration. If you want to always send the call to a predefined number you’d need to have it sent to that number by either the sending or the receiving end. The connection between has no way of making that alteration, it has to be done on the PBX side. As this is outside of this community’s area of expertise you’d better search for that information elsewhere.