What is Direct SIP with IP-PBX? Direct SIP is the documented and supported way in which Microsoft Office Communications Server exchanges voice calls with third-party on-premise devices such as SIP/PSTN gateways and IP-PBX. In Direct SIP, an Office Communications Server’s Mediation Server is directly connected to a SIP/PSTN gateway or an IP-PBX. Microsoft provides the Unified Communications Open Interoperability Program (OIP) for the qualification of third-party solutions for interoperability with Microsoft Office Communications Server. Under the OIP, Direct SIP is based upon common industry standards (SIP over TCP, RTP, G.711…). Direct SIP with a SIP/PSTN gateway enables Office Communications Server to exchange calls directly with the PSTN, as well as with virtually any TDM PBX, and (via back to back SIP/PSTN gateways) with virtually any IP-PBX.

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