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Hybrid PBXs

You can begin to enjoy many of the advantages of VoIP without a complete replacement of your current PBX by taking a hybrid approach. David Jacobs explains how in this tip.

This tip originally appeared on, a sister site of

The advantages of Voice over IP (VoIP) have become clear over the past few years, but the thought of ripping out and replacing a functioning phone system is a frightening prospect. Hybrid PBXs support both traditional Time Division Multiplexed (TDM) phones to VoIP, easing the transition from TDM to VoIP.

Suppliers of traditional PBXs such as Avaya and Nortel offer add-on boards or gateways that enable you to turn a traditional PBX into a hybrid PBX, providing a step by step path to VoIP. You can begin to enjoy many of the advantages of VoIP without a complete replacement of your current PBX.

VoIP creates savings in multiple ways:

  • Allows calls to be routed between company sites over the data network, thereby avoiding phone company charges.
  • Reduces and simplifies office wiring. VoIP enables you to run a single set of wires to each desktop to support both data and voice.
  • Enables unified messaging so voicemail can be accessed from a phone or from a PC.
  • Enables access to the PBX from soft phones so employees can route calls through the PBX from home or when traveling.
  • Eliminates the need to update the PBX when users move from office to office since the phone number moves with the IP phone set.

Recommended steps in implementing VoIP

A first step in the transition from TDM to VoIP often consists of adding an interface to the PBX to connect it to the company data network: an easy first step because there are no changes to connections to the desktops. Calls from one company site to another are then routed over the data network, avoiding phone company charges. A TDM interface to the public switched telephone network (PSTN) remains in place to handle calls outside the company. If problems occur with the newly added interface, intra-company calls can be temporarily routed over the PSTN.

The next step can take place when a new block of offices is added or an existing office area is remodeled. Adopting VoIP for these offices means saving the expense of running phone wiring. You will wire the area only for the office data network, which will need to be installed in any case. VoIP means that there is no need to run traditional phone wiring to the new offices.

Benefits of a hybrid approach

Hybrid PBXs offer the same set of capabilities to TDM phones and to IP phones. This means that features such as unified messaging are available to users of TDM phones as well as users of IP phones. As soon as IP capabilities are added to the PBX, all users will be able to take advantage of what have often been thought of as VoIP only features. This ability is not limited to users with IP phones on their desktops.

Adding IP capabilities to the PBX also enables all users to connect to the PBX using a soft phone. Calls from a soft phone are handled in the same way as a desktop phone in the office area, so users, while at home or traveling, can check voicemail, make internal calls or take advantage of negotiated long distance rates.

Making the switch to VoIP does not require you to throw away existing phones and buy new IP phones. If you want to avoid the cost of new phones, existing phones can be connected to the IP network through Analog Telephony Adapters (ATA). An ATA is a device with a LAN interface on one side and a traditional phone plug on the other side. It translates signals so that a traditional phone appears to the PBX to be an IP phone.

As you gradually migrate to IP phones, the need for administrative changes to the PBX during office moves disappears. As long as you have traditional phones, you will continue to need to make changes to the PBX to reflect office moves, but as more and more office areas are equipped with IP phones, the administrative load will decrease.

Eventually all TDM phone equipment will be retired, but risking an entire corporate phone system during a complete upgrade is too risky for most telecommunications staffs. Hybrid PBXs offer a much safer way to enjoy the benefits of VoIP while reducing risk to a reasonable level.

David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.

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