I'm currently trying to write a native iPhone Lync client. I've been mucking around the internet to find the appropriate SIP stack library.
Would you guys recommend LibSIPE as the base library and is it possible to port the entire library to IOS ?
A SIP stack won't help you at all with the OCS protocol. Yes, it uses SIP messages, but that's basically where the similarities end.
It should be no problem to compile SIPE code for iOS. It already compiles for Adium on Mac OS X -> XCode can grok it. It has been compiled/running on Debian5/armel, N900, Fedora/ARM -> can be (cross-)compiled for ARM targets.
I guess the only thing which is likely impossible on iOS will be media support, i.e. audio & video calls. That is based on gstreamer & farstrem and I highly doubt that those have been ported to iOS.
The only question is: are you willing to spend the effort? You will have to add a new backend to hook up the independent SIPE core with iOS (transports, timers, … see src/api/sipe-backend.h & sipe-core.h). The latest git HEAD contains 3 backends (libpurple, Miranda & telepathy ), so there should be enough example code flying around to get you started with a new backend.
I'll be frank with you: I personally will only answer questions from a developer who will contribute his code changes directly back to the SIPE git repo. I.e. if you fork off your own private copy, e.g. for commercial gains, then you are on your own.