Lync is built to integrate with existing systems, but there are a few things you'll need to check to determine how you should set up the integration.
On the Lync side of the story you'll need the Mediation Server role, which is required in this scenario. Then you'll need to decide if you will
collocate the mediation server role on the Lync server or not.
On the PBX side of the story, you should first check if your existing PBX and the software version it uses is compatible for direct SIP connection to Lync. You can check this on the
UC interop Program page.
If this is not the case, you'll need to use a gateway between the PBX and the Lync Setup to make it work. If the PBX is not on the supported list, it means that they both speak SIP but are using different "flavors/dialects" which the gateway understands
and will translate between both devices.
Technical Specialist Microsoft OCS & UC Voice Specialisation - http://www.uwictpartner.be
