Update 9/2/207: For complete information, please refer to OCS_VoIP_Guide.doc from the OCS RTM documentation. The below information is very high level only.
It depends what your telephony infrastructure supports:
My PBX supports CSTA over SIP (less common)
You can integrate directly with OCS with Remote Call Control (RCC) without the need for a Mediation Server. Depending on your capacity requirements and topology you may want/need one or more dedicated servers to handle the SIP routing. However, you will need a Mediation server and media gateway to make phone calls from Communicator.
My PBX does not support CSTA over SIP (more common)
You can use the Enterprise Voice and/or RCC capabilities of OCS, but you will need a third party media gateway to do so. See http://forums.microsoft.com/Ocs2007/ShowPost.aspx?PostID=1544856&SiteID=57 for links to supported hardware, and keep in mind that this list will be expanding as time goes on.
