Although Microsoft Lync Servers can interact with PBX using specific gateways and additional servers, that configuration is not always the best solution. The installation is complex, and requires important architecture changes and knowledge. While that install may be the key for large and complex installs, is not practical for small installations, or when you need to implement the features to a subset of users. More important, Enterprise Voice does not work currently on Microsoft Online Services, so if you are cloud based, a plugin for lync is your only option available. Additionally, you are limited to basic voice/video features, while SimpLync adds also powerful call center features.