I think I may have posted about this before but I couldn't find it, so since I was doing a clean install of Ubuntu 12.04 on one of my laptops this weekend I figured I'd write it up again.
If you're on Ubuntu 12.04, you use Pidgin (which you should -- Empathy sucks), and you need to connect to MS Lync (formerly known as Office Communicator), thankfully you can simply install the SIPE plugin for Pidgin:
sudo apt-get install pidgin-sipe
In my case I also had to specify this as the User Agent to get it to connect:
UCCAPI/4.0.7577.314 OC/4.0.7577.314 (Microsoft Lync 2010)
There's one additional wrinkle on Ubuntu 12.04, because even after installing the SIPE plugin and setting the user agent, you may see "Read error" when you try to connect.
You're never alone on the Internet -- other people have run into this too, and the solution is simple enough, but since I didn't want to have to do that every time I launched Pidgin I put that in a bash script and changed the Pidgin launcher to point to this script.
You can put this anywhere so long as it's executable, but here's the script:
I feel like a bad nerd by admitting that I have no idea what that does and didn't take the time to look into it since it solved my problem, but there's the solution that works for me.