If your budget spreadsheet proves the only choice is to upgrade your primary network to provide unified messaging...
over a converged network you will soon bump into a new breed of networking device called the Session Border Controller (SBC). These intelligent devices look at first glance to be VoIP firewalls because they are installed at the edge of the network. However, they also provides session management capabilities to regulate and oversee real-time traffic flows within IP networks. The SBC addresses a number of critical issues such as network security, signaling interoperability, call admission control, service quality, and session routing. In effect, it secures the VoIP network, helps you manage VoIP traffic and other forms of real-time traffic, and enables the easy addition of new services and applications.
Again you will need to allow some budget dollars for training, as most of your networkers won't be familiar with the technology, although being based on well-known IP technologies, the learning curve isn't all that steep, unless they only learned their skills at a single vendor college. Any networker educated in the fundamentals of IP will soon come to grips with the converged version of the beast. And they're going to need to be up-to-speed because even if you aren't ready to jump you will soon be getting requests from trading partners and application service providers to mesh your network with theirs. At the very least you need the in-house skills to just say "no" until you're ready to jump.
The ubiquitous IP-based network is being made to do things it was never designed to do well, much as Ethernet has been forced to be all things to all networks. But it's what we all know, and we've got truckloads of money invested in our existing networks, so we need to find ways to make them do what our users demand. Session management technology enables you to tackle these challenges, but it only provides half the answer. Networkers trained in the engineering of the session layer, are needed to help design, build and maintain converged networks.
Don't be afraid to talk to your favourite network vendor about these issues. Although their training will focus on the specific commands and interfaces built into their own products, which is going to come in handy anyway if you've bought their kit, these new devices are all based on published standards, so you won't be hearing radically different songs from each vendor. The interpretation might change but the melody and the lyrics will remain the same.