Can we have SIP trunking with our TDM PBX?

By Robin Wilding
Updated: February 08, 2011

So you quasi-recently invested in a TDM PBX only to find out that telephony is moving at the speed of light and that TDM PBX system has already become outdated. Many companies invested in TDM PBX back in 1999 and still have years of life left in the hardware but are dying to upgrade to a cheaper SIP trunking solution. These companies are now wondering if they can salvage their TDM investment but still add SIP trunking to their existing infrastructure. Good news.

It is possible to use your legacy TDM PBX solution alongside SIP trunking, while still maintaining the benefits and features of both.

How the heck?

The process will be quite simple if you have available TDM trunk ports…but most likely you do not. To route a new SIP trunking solution through your TDM you need to make a “drop and insert” style installation by placing a media gateway into the TDM trunk circuit. You will need to insert the media gateway in-between the TDM and PBX so that one side of the gateway is facing the TDM and the other side the PBX. The gateway will essentially go unnoticed as it will simply route traffic from one appliance to the other, but it is an essential ingredient.

Now you need to setup your SIP trunking solution and terminate it along with the TDM on the media gateway that you have just installed. When you have successfully completed this circuit you will have both the TDM and SIP terminating on the newly installed gateway. Routing tables will now be cleverly shuffling the calls between the circuits.

This new solution should also provide a failsafe in that if either the TDM or SIP fails the service will roll-over completely to the service still running. If both the TDM and SIP fail then you are stuck.

This solution however is resting on the fact that your PBX is SIP-ready. If by chance your PBX is not SIP-ready as you are running a key system or traditional analog then you will need to install an integrated access device.

This solution is surprisingly easy to install and configure but will give you a few more years of use from your TDM before you switch entirely over to a SIP trunking solution.


You already laid out a whack of dough for the TDM solution so it is important that the integrated SIP Trunking solution not require too much outlay. The initial costs to integrate your existing TDM PBX with an SIP trunking solution are minimal. The important ingredient that you need to purchase is a media gateway, which should cost between $1,000 to $5,000 depending on your data volume and the brand of hardware that you choose. Then you have the cost of the SIP trunk implementation, which will vary depending on the solution you choose.

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