Linking Asterisk to a Legacy PBX


I’m after information about the different ways (if there are more than one) to link an asterisk box to a Legacy PBX. At this stage I’m looking for general ideas, not specific to one make and model.

I’m an IT consultant working in a regional area of Australia. Most of the townships around here have ADSL so thats not a problem (We are also working on Wireless ADSL for those further out of town). Our own internal PBX system is about to be completely replaced by our Asterisk box equiped with a TDM400 card with 4 FXO ports, so we don’t need any intergration.

But we are aiming to become the VoIP experts in the area as no one else is. Some of our clients (large hotels etc) have expressed great interest in the cost savings that VoIP has to offer, for both internal use (distributed sites linked by wireless) and external calls (much cheaper rates with VSP’s than telcos over here).

However I’m sure most of our clients will want to keep there existing PBX system, but would like the flexibility and savings that a VoIP system could offer.

What I’m looking for is any docs, info, sites that can explain the basics of interconnecting an asterisk box to a legacy PBX for:

  1. VoIP gateway to make cheap VoIP calls from existing PBX
  2. Adding extensions to remote locations utilising the wireless ethernet network.
  3. Any other reason we can think of.

If anyone can point me in the right direction and shed some initial light on this, it would be greatly appreciated!

Kind Regards,

Here is some info for you … ntegration

Hi Rusty,

Thanks for that. I actually found that link after posting, but its mostly information on the config settings, at this stage i’m interested in what cards and hardware are needed to link up to a legacy PBX. Obviously the first thing that comes into mind is a TDM card with FXS ports comes to mind, but that isn’t helpful if you have more than 4 lines.

Any info on the hardware would be greatly appreciated.


There could be several ways to do it. It really depends on what side of the pbx asterisk is connected to and what kind of lines your pbx handels?

For instance if your asterisk box is setting between the pbx and the pstn. you originally had a T1 from the pstn to the pbx. Then you could use a dual T1 card in asterisk connect one side to the pstn and one side to the pbx all calls go threw asterisk then you could route local calls to pstn and longdistance to a voip provider. or if you were using pots you could use the newer TDM2400P and have 12 fxo and 12 fxs ports

Just to add my 2 cents.
I intergrated my PBX to Asterisk with a PAP2.
I have 2 POTS inputs on my AT&T Partner. I have only one incomming POTS that Asterisk now handles, and forwards to the PAP2 -> Partner as needed.
This has given extra functions as I can now feed Landline and VoIP to the PAP2 and /or other SIP phones on my network.

Thanks all for your replies.

I have spent the day researching, and I think I have come up with what I’m needing.

One client has an existing legacy PBX that has 3 Telstra OnRamp2 ISDN connections. OnRamp2 for those of you who don’t know is a BRI 2xB channels 1xD channel. (And I’m in Australia)

So to have an Asterisk box inbetween their legacy PBX and the Telco, I need to have two four port BRI cards, such as the Junghanns QuadBRI. One of these cards would be configured as NT mode and the other as TE mode. One connects to the three ISDN OnRamp2 connections, and 3 of the other QuadBri connect to PBX.

Does this sound right?

Damn BRI cards are expensive.

Sounds like your on the right track. But I could be wrong never used that type of setup