Combining Asterisk with another PBX

We currently have an existing PBX system in our office which we’ve been unsuccessfully trying to expand.

At the moment this PBX system uses a external VOIP provider for outbound calls, and ISDN for incoming.

What we wanted to achieve was that outbound calls to mobile telephone numbers would travel out using a SIP to GSM gateway, and the landline calls would continue using the existing VOIP provider.

It turns out however that our existing PBX cannot connect to multiple SIP trunks at once, so we cant have it connected to both the GSM gateway and the main voip provider at the same time.

What i’m hoping is i can slot asterisk in between the two. I know asterisk can happily handle multiple trunks and has plenty options for routing the outbound calls to the appropriate trunk.

What i’m unsure about is how i setup Asterisk to act AS a trunk for the existing phone system, or what this mode of operation would be called.

Any tips or pointers on how to go about this would be much appreciated.


Can you setup a trunk between the old phone system and Asterisk?

What sort of traffic volumes are you expecting?

i would setup Asterisk and let it register to your voip-provider.
You make an account for the gsm gateway

Then you make a simple dialplan that sends calls to landlines to your voip-provider. Mobile numbers you send to the gsm-gateway.

You find an example for the gsm gateway at

You can even specify in the diaplan that when the call fails because of a problem with the voip-provider/gsm gateway, it calls out over the gsm-gateway/voip-provider.


Well i think this is what i need to do, i’m just not sure how to go about it. Is there a specific way to configure asterisk such that it acts like a SIP trunk the old PBX can connect to?

Venalbc: currently we’re seeing peaks of around 2-3 simultaneous calls going out over the sip trunk. I’d suggest another 1-2 over the GSM gateway once thats running.

What is the model of your PBX ? if you have SIP trunk with VOIP provider you can have sip trunk with asterisk as well .
If your PBX doesnt support SIP trunk you can trunk your PBX with asterisk through E1/T1 Cards too .

Hi there,

Our current PBX is an LG IPECS 1200.

Does anyone have any pointers on connecting such a system?


LG IPECS devices support SIP . you can connect it to asterisk through SIP trunk . ofcourse you can connect it thorugh E1/T1 pri as well .