OCS 2007 R2 won't replace your PBX just yet. However, their latest R2 version adds the ability to do direct SIP trunking, thus bypassing the need for an IP-PBX.One drawback however is that Microsoft only supports direct SIP trunking with two providers, namely Global Crossing and Sprint. Well that's pretty lame, considering their are dozens of decent SIP trunking service providers and probably hundreds across the entire world.
Fortunately, Mike Stacy an OCS 2007 guru, over at Evangelyze Communications has some products that enhance OCS 2007 R2 functionality. One such product is SmartSIP which launches tomorrow. According to Mike, the first dot release due next month will add the capability to use standard SIP phones with OCS. Currently, you have limited options namely Tanjay or Snom phones, but with SmartSIP you can use a Polycom IP phone, an Aastra IP phone, or dare I say, a Cisco IP phone connected to OCS 2007 R2. 
With the Cisco SIP firmware load of course.
Not only does it enable SIP endpoints, but it also enables you to use any SIP trunking service provider. Essentially SmartSIP enhances OCS 2007 R2 so it can be used with any SIP trunking service provider (UDP or TCP) effectively eliminating the need for an IP-PBX for trunking, while at the same time allowing you to leverage your existing IP phone hardware investment. While OCS 2007 R2 may not be a PBX killer by itself, by adding SmartSIP, you can kill the PBX! This functionality (any SIP phone + any SIP trunking service provider) should have included in the OCS 2007 R2 release. I suppose there is always Microsoft OCS 2007 R3! 
SmartSIP Features:
SmartSIP Features:
- ITSP Solution for OCS 2007 R2
- UDP and non-UDP ITSP support
- Multi-SIP Trunk support per single OCS Mediation Server
- Improvements to OCS 2007 R2 platform
- ENUM support
- NAT Traversal
- AD phone number matching for extension/DID
- Support for generic SIP PBXs and endpoints/devices
- Virtual gateway service
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This is glorious!