1) SIP Trunking: this is a solution for the interconnection of an IP-PBX system with the PSTN network over an IP trunk carrying SIP signaling and RTP media (with a range of options). Microsoft participates in the establishment of standard solutions in this domain via the SIP Forum. There is a specific program in the OIP for qualification of service providers.
2) Direct SIP: this is a solution for interoperability of OCS with a third party telephony solution, typically Gateway or IP-PBX. It also uses an IP trunk carrying SIP signaling and RTP media. Microsoft has specific programs in the OIP for qualification of third party gear. The Direct SIP requirements may involve call flows that are more complex/different from the SIP Trunking, with private number manipulation, call transfers, holds on both sides, etc.