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Strategic Considerations for Switching from a Legacy PBX to Communications Server

Microsoft Office Communications Server 2007 and Microsoft Office Communications Server 2007 R2 will reach end of support on January 9, 2018. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.

When making long-term plans for your organization’s IT strategy, the elements that make up your voice system should be carefully considered. This article explores the question: Should you retain your existing PBX or replace it with a software-based solution?

Author: Adam Jacobs

Publication date: July 2010

Product version: Office Communications Server 2007 R2

When you consider the components in an enterprise IT strategy, the voice elements should not be thought of as insignificant. The question of retaining your existing PBX or replacing it with a software-based solution such as Microsoft Office Communications Server is important and one that needs to be carefully factored into your long-term IT strategy.

Chances are many of you have already invested thousands of your local currency in telephony equipment, and simply abandoning your current configuration is not going to be an option. At the core of this investment often lies PBX. As an information technology strategist, one of your key decisions could be whether to take the traditional vendor route. In this case, a physical PBX (“hard” solution), or take the plunge into a software-based (“soft”) solution. For the minority who has made the move to unified communications (UC), you may have chosen to replace your PBX with a voice gateway. As a result, your switch to soft telephony is for the most part complete. That said, for those who have not yet made this switch, it is a decision that should not be taken lightly. There are several factors you need to consider, such as the following:

  • A five-year cost comparison of hard versus soft solutions

  • Whether a software alternative should be considered if you choose to replace your PBX

  • Whether to move away from traditional vendors and consider Communications Server on the basis that UC is about the user experience and tighter desktop integration

  • How to address subsequent gaps in telephony expertise within your organization

Imagine the following scenario: You have a site that is in development that requires a resilient, on-premise voice solution for 300 users. You have already adopted several Microsoft solutions, which is common in today’s enterprise. You go to submit your proposals and, in line with your business, you construct a five-year total cost model that looks similar to Table 1 and Table 2.


All costs are approximate, exclude U.K. VAT and associated consultancy fees.

Table 1. 5-year PBX costs

5-year PBX costs

Table 2. 5-year Communications Server costs

5-year Communications Server costs

As shown in Tables 1 and 2, the total cost of ownership based on a five-year time frame for a soft solution is substantially lower, over 50%, than a hard solution from a market-leading communications vendor. While the Communications Server solution in itself is less expensive, the biggest cost reductions are personnel and the opportunity to maximize the use of existing in-house expertise, which can produce a saving of as much as 70%. This is based on the premise that you already employ Windows Server and Exchange Server administrators; therefore, your organization will require only an additional ½ a person.

The budget for a Communications Server solution is based on the design illustrated in Figure 1.

Figure 1. Logical Layout for Solution

Logical Layout for Solution

In my previous article (Unified Communications and Its Impact on IT and the Business from Delivery to a Year and Beyond), I mentioned that Office Communications Server 2007 had taken me on a journey from immobile telephony to an entirely location-independent communications solution. The co-existence of soft and hard solutions allowed us to make our first in a series of steps toward completely replacing our PBX. In short, an overnight migration is unrealistic, and I would not suggest it, unless you are faced with implementing an entirely new site.

So why make the choice at all? When I introduced UC into my workplace (a financial services organization with over 300 employees) in early 2008, the benefits of a telephony-integrated desktop were noticed by the majority of users almost instantly. As with many enterprises, we use Windows and several Microsoft applications. What became clear is that the way in which the Communicator client integrates with other Microsoft products has been a key to driving adoption and, ultimately, a successful deployment. Therefore, the thought of a completely software-based solution becomes very attractive and permits further integration opportunities moving forward. Microsoft permits the use of this extensibility in Communications Server through its Unified Communications Managed API (UCMA).

I also mentioned in my previous article that IT professionals are starting to become just as responsible for voice provisioning as they are for any other IT Infrastructure component. Unfortunately, in many businesses this is still not the case, and as a consequence there appears to be a lack of cohesion between these two worlds. One of the benefits to be gained by a soft solution may be the move to a common UI, whereby existing system administrators will be able to take on this new system with little need for training.

When taking into account these gaps in telephony expertise, you might want to consider a move to hosted services. UC telephony, unlike many other end-user based technology enhancements, can provide a wealth of extensibility for your back-end voice infrastructure. There are currently two common approaches:

  • SIP Trunking   This is similar to a SIP connection (which is established between your Communications Server Mediation Server and your chosen gateway or PBX). However, the idea is that the move away from your PSTN takes place by using a remote or third-party network. Industry feedback so far has shown that an overnight migration to a service of this nature is ill-advised due to network capacity and resilience considerations. A more strategic approach, which would deliver significant cost benefits in the short term, could be routing higher rate calls through lower cost carriers, which is often referred to as least cost routing.

  • Hosted PBX   Hybrid solutions (a combination of on-site and off-site technology provision) could also be an option after your voice infrastructure is, for the most part, location-independent. By shifting your core telephony components and leaving infrastructure such as domain controllers, Exchange, and Communications Server in your environment, you may be able to consider rationalization of voice or data networks, power, and space. In addition, you may find that your hosting partner can offer better resilience or lower rates based on the fact that the service is being offered to many customers.

    In some territories, mobile carriers are assisting businesses in making a complete end-to-end UC transformation by offering a hybrid solution that delivers fixed-mobile convergence. I refer to this as the “last mile” of UC—the capability to merge your fixed-line voice mail or direct inward dialing (DDI) into the mobile network, without picking up additional call costs incurred by using fixed-to-mobile call redirection.


In summary, there are many benefits to using UC or a PBX. Table 3 lists some of the high-level pros and cons for both solutions.

Table 3. Summary of pros and cons

UC/PBX Pros and Cons

Finally, this is obviously not a decision that can be made, or indeed implemented, overnight. If choosing to adopt UC, you may want to consider a phased delivery across a number of years due, not only, to initial financial outlay but also a method whereby substantial changes do not upset the stability of your existing IT service. Before you decide which route to take, it’s important that you think about how these capabilities fit into your current or future UC strategy, taking into account your end goal, and that you prioritize functionality based on the needs of your end users.

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