Bridging PBX to VoIP using a hardware gateway or VoIP enabled router

The simplest way to bridge between VoIP and conventional telephony systems is by installing a hardware gateway, or a VoIP-enabled router.

VoIP-enabled routers typically offer between one and four analogue telephony connections, or FXS ports in the telecoms jargon. These ports will usually terminate in a US-style RJ11 phone socket, and will connect directly to an analogue phone or other compatible phone equipment.

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Although routers from manufacturers such as Speedtouch, Draytek or ZyXEL offer such ports, and will work well with hosted VoIP solutions, they are designed primarily for the teleworker or micro-office markets. Small VoIP adapters, with one or two analogue ports, are an alternative for small offices where companies would rather not replace routers.

Businesses that need to bridge multiple VoIP trunks to an existing phone system will need one, or more, dedicated hardware gateways. Gateways will usually offer at least four and often eight or 16 analogue FXS ports. (FXO ports, which might also be present on VoIP gateways, are used to connect local IP phones or other VoIP hardware to analogue or ISDN trunk connections). Some gateways convert VoIP traffic into ISDN signals, allowing connections to ISDN-based PBX systems, although these tend to be more costly. Gateway manufacturers include Linksys (part of Cisco Systems), Epygi and Grandstream.

 The configuration interface for adding a user's IP phone - In this case for Viatel's hosted VoIP service.
With suitable wiring, it is perfectly possible to connect FXS ports from a router or gateway to a client’s PBX. The level of technical expertise needed to do this will depend on the age and model of the PBX, whether it has spare trunk connections — or whether PSTN trunks can be reused for VoIP — how the PBX is wired into the phone network and whether it is covered by a maintenance contract.

Smaller PBX systems, especially newer designs, have direct RJ11 sockets for incoming lines (ISDN2-based systems typically use an RJ45 connection, and these are almost always socket based). Other systems might take their lines through a separate cable box, using punch terminals hardwired into the telecom provider’s master sockets. Such systems are not designed for user maintenance, so IT consultants will need to enlist the support of a PBX engineer well versed in maintaining that particular system.

An engineer will almost certainly be needed if a PBX needs a hardware upgrade or modification, such as the installation of a new trunk card, or reprogramming, in order to accommodate the new, hosted VoIP lines.

Hosted VoIP provider,, offers a simple interface for analysing outbound calls.
Some more modern PBX systems, such as Nortel’s BCM series or BT’s Versatility, allow direct connections to VoIP over an Ethernet connection. But again, this is a task best left to a qualified engineer; this will usually be a requirement if the system is under a maintenance contract. However, this is an issue that most hosted VoIP providers deal with on a regular basis, so they should be able to introduce you to local, qualified PBX specialists.

Despite the complexity, the advantages of bridging to a legacy PBX system are twofold. Firstly, it allows customers to prove the concept of VoIP, by running VoIP and conventional phone lines alongside each other. VoIP can be used to add capacity, for example, without incurring the costs of additional phone lines, or to take advantage of cheaper calls, especially for international numbers or to mobiles.

 Setting up hunt groups for inbound calls using hosted Centrex running over VoIP.

The second advantage is that a hybrid approach allows companies to make use of features inherent in hosted VoIP — such as the ability to use a softphone at home or on the road for work calls, or cheap or free inter-site connections with staff having only one contact number to give out — alongside existing handsets and extensions and infrastructure such as voicemail or receptionists’ stations.

For companies that make a significant number of internal calls, using existing TDM extension wiring takes the load off the LAN. Nor should you underestimate the cost of adding additional Ethernet ports to a company’s offices, in order to support VoIP. Gateways allow companies to phase in such upgrades, as the business demands it. The customer can then make the full move to hosted VoIP, when the PBX is at the end of its useful life.

Architecture of a hosted VoIP installation

Link to a Relevant FeatureClick here to go to the main feature - Unplugging the PBX for a VoIP installation


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