If you have never heard of SIP Trunking, here is a quick high level explanation. OCS R2 includes support for SIP Trunking, and in a nutshell this allows OCS R2, and therefore your R2 users, to have voice interoperability with the public phone system (PSTN) without any IP-PSTN gateways or other PBX equipment. A SIP Trunking Service Provider is required and used instead.
A more technical definition of this feature is that the SIP and RTP protocols are used to pass telephony traffic to and from the edge of your OCS deployment over an IP connection. This is big-win for OCS: PSTN voice interoperability without the burden of deploying and maintaining hardware to make it happen.
The big driver for adopting SIP Trunking, is cost savings:
1. cost savings on IP-PSTN gateways and possibly other PBX equipment
2. cost savings on PSTN trunks expenditures
3. cost savings on long distance by enabling dynamic call routing; policies can be configured to route calls based on location, time of day, etc…
More details about the OCS R2 SIP Trunking feature can be found here: