When you say "work the ISS," I ask "which mode?"
There are two primary ways to "work the ISS."
1. FM voice crew contact. This is pretty rare these days. The crew has other things to do for leisure. They do it for special events and occasionally in their free time. It uses 2m FM simplex, split channel. It's workable with a modest omni antenna, but a 2m yagi (2-3 elements) would be better.
2. FM packet exchange with another terrestrial station. Essentially in this mode, you use the ISS as a packet digipeater. 2m FM, single channel.www.issfanclub.com
<- Make that your friendly site for ISS information.