If the PBX in question is IP capable (hybrid or native), it can integrate with OCS RTM and R2 via Direct SIP with an intermediate media gateway. This is provided that it is certified under the Microsoft Unified Communications Open Interoperability Program (OIP) for enterprise telephony infrastructure.
Otherwise, the classic method is to use Back-to-back IP/PSTN Gateways to bridge the OCS Mediation Server with the legacy PBX. Alternatively, a basic gateway can sit between a Mediation Server and the PBX to connect the 2 systems. A hybrid (collocated) gateway e.g. Audiocodes or Dialogic is another option where the media gateway appliance and OCS Mediation Server role both reside on the same box.
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