Source: No Jitter
"Intelligence is based on how efficient a species became at doing the things they need to survive." -Charles Darwin
Back in the day, it was extremely common for a company to standardize their communications infrastructure on a single vendor. For instance, if I were to walk into a "Northern Telecom shop," I was pretty confident that I would see a Meridian One for call processing, Meridian Mail for voice mail, Meridian ACD for call center routing (the term Contact Center had yet to be coined), Meridian MAX for call center reporting, and a slew of M2616 digital telephones. The same would hold true for Lucent, Rolm, Mitel, etc. There was a sameness to the PBX that started at the core and extended all the way to the handset.
Not only did communications vendors have their own unique software, but everything ran on proprietary hardware. The concept of rack-and-stack servers was completely foreign and, for better or worse, an enterprise had no choice but to put all their eggs into a single basket.
Of course, that was then and this is now. Much of that proprietary hardware is history and most software now runs on off-the-shelf servers. Better yet, virtualization has been widely adopted and a great deal of an enterprise's communications system lives in a virtual server farm. Proprietary gateways still exist, but as the world continues to move to SIP, they too will one day be put out to pasture.
As communications vendors and their products have evolved, so have the people who've traditionally brought them to you -- the ecosystem of business partners that sell, install, configure, and maintain most of the communications systems out there today. The old business models no longer work in a world where software and integration services overshadow hardware.