I have spoken to a number of Cisco VoIP customers who questioned how to proceed if they want to go with Microsoft but aren't ready to move to Nortel.
I answer these questions in the following way:
1) Recognize that Cisco and Microsoft are direct competitors in the UC space (Chambers and Ballmer agree on this point) with very different UC visions (network vs software centric). Therefore expect Cisco and Microsoft to drag their feet on interoperability today and in the future. As one Microsofter observed "Cisco was the last major PBX vendor to offer remote call control with us". Even with interoperability, Cisco will continue to promote their agenda: one customer observed "why else do I need a Cisco Presence server when I want OCS to be my single my presence engine?"
2) Cap investments in Cisco VoIP, and get on a better path to software-centric UC. One approach promoted by Microsoft is to use OCS telephony, wherever it meets the needs. Another approach is one used by one large petroleum company: they are deploying Nortel IP Telephony in three regional data centers as integration points into LCS/OCS and interoperability point into their large non-Nortel installed base of PBXs. This is a slight variation of Microsoft marketing speak "UC, as you are".
3) Either embark on roll-your-own deployment or turn to a partner/SI. Don't leave it to Cisco because their bias will not work in your favor.
4) However you decide to evolve your IP telephony environment, down the path, you will be able to take advantage of Nortel OCS value added capabilities, such as our reservationless conferencing system tightly integrated with OCS, or our integrated branch solution embedded with Microsoft UC technology.
That's my thoughts. And yours?