[Asterisk-Users] Linking asterisk to an existing small office PBX

[Rich Adamson]

> I've done some reading on the wiki and read some of the mailing list 
> archives, but can't see anything on this. I guess this means I'm either 
> searching on the wrong thing, or have totally the wrong idea... Can anyone 
> suggest if the following is possible?
> 
> Currently, our office has a 24 analogue extension PBX, and 2 ISDN lines
> providing it with external connectivity. We have several analogue
> extensions spare, but no capacity to add fancier connectors to link to an
> asterisk system (as most of the PBX linking guides detail). All our phones
> are bog standard analogue ones.
> 
> We'd like to use an asterisk system to allow some calls to be routed out 
> via a VoIP gateway. We'd also like to allow some inbound SIP calls to be 
> handed to the PBX.
> 
> I was considering building an asterisk system, and adding a few FXS cards
> to it. I'd plug these into the spare analogue extensions on the PBX (is it
> FXS, and not FXO I'd need when connecting to a PBX?) If people wanted to
> make a call using a VoIP gateway, they'd dial one of the asterisk
> extensions. They would then be connected to the asterisk box by the PBX,
> and could dial the real number they wanted. Finally, asterisk would
> connect them to an external SIP gateway, which would do something useful
> with the call.
> 
> Additionally, I'd like people working from home to be able to connect via 
> SIP to the asterisk box, and then have their calls routed to the PBX down 
> an analogue line.
> 
> Is this possible, and is it even a desirable setup?

Yes, all of that is possible with lots of folks already doing it.

The extension appearances on your existing pbx are "fxs", therefore the
mating interface on asterisk has to be an "fxo". The digium TDM04B is
one example of a 4-port pci card supporting 4 fxo interfaces.

If each of those four fxo interfaces were connected to unused extensions
from your old pbx, incoming voip calls (via the Internet) can be routed
to any of those four fxo ports. Likewise, an existing pbx user could
dial "8" as an example, and be sent to asterisk via those same four ports.
You would need to be able to set up the dialplan in both the old pbx
and asterisk to handle the exact dialed digits the way that you want.

It is highly unlikely the old pbx can be made to forward callerid
numbers to asterisk, etc.

As time and budget permit, you could migrate to using SIP phones on the
asterisk side as well, displacing the old pbx analog phones.