As part of OFA, I've been making calls into a number of states, and I have to say that Wisconsin is one of the more complicated I've called.
On the plus side, I probably just guaranteed two votes for Feingold and Barrett by spending 20 minutes on the phone with someone that was planning on vote but didn't have a lot of details and didn't know about early voting.
Some crucial links if you're going to call into WI, based on my struggling with bad links for the last 20 minutes:
Early voting in Wisconsin main page
Wisconsin county clerk directory
Note that the directory link has both county clerks and municipal clerks. They don't appear to overlap, so you may need to check both.
Hopefully this helps, and good luck!