Asterisk and Legacy PBX's

Greetings everybody!

I hope someone can give me some insight.

I work for a Mexican government agency. Our main faciltiy is running a 15 year-old, very reliable BOSCH PBX, handling 500+ extensions. We also have serveral secondary facilities, each one running their own PBXs (different brands, none of them supporting VoIP). In total there are about 1,800 extensions spread throughout all our facilities.
Most of our legacy PBXs interface to the PSTN through E1 ports.
We also have a VPN linking all our facilities, with enough bandwidth to support VoIP.

Here are my questions:

1. As a first pahse towards inter-office VoIP, can Asterisk be used to economically build a VoIP bridge between our legacy PBX’s?

The idea would be placing on each faciltiy an Asterisk Server sitting between the PSTN E1 ports, the VPN and the PBX’s E1 ports. The dial plan would be built in such a way that it can route inter-company calls through the VPN, while external calls are forwarded to the PSTN.

2. Can this bridge be built by using a pair of E1 interface cards available on the market, one for interfacing to the PSTN and another to the PBX?

3. Is this a sound strategy?

I really appreciate any advice over the matter. I’m a newbie, but I’m convinced Asterisk is the way to go. :smiley:

Sergio Gerardo

What you describe is a good step towards full VoIP using Asterisk. Asterisk can be used to bridge your phone systems. The main thing is to ensure that voice has sufficient commited bandwidth by using QoS based network infrastructure (Asterisk will send IP packets with a specific Type of Service [ToS] which switches and routers need to pick up and forward correctly. The other part that may give you some problems is passing the CLI from the phone system to Asterisk. This can be a bit of a trial and error thing with some phone systems. You could do:

Legacy PBX (E1) --> (E1) Asterisk (IAX2) --> Network --> (IAX2) Asterisk --> (E1) Legacy PBX

That would work well. You could also use the Voicemail facilities in Asterisk in this way if your legacy PBX does not have voicemail or connect IP phones via Asterisk to link into the Legacy PBX systems.


Thanks for your excellent reply and your excellent advice.
We’ll probably begin trials next weekend, wish us luck!

Sergio Gerardo


Great news! I hope that you really get a lot out of your Asterisk implementation. PM me if you need any advice!


If you want you can implement CONFERENCE featuer for your users using Asterisk PBX.