Leonard Boswell 202-225-3806
Charles Grassley 202-224-3744
Tom Harkin 202-224-3254
The numbers above are for their D.C. offices. I call during business hours. I tell the person that answers the phone that I am a constituent and I want to give my opinion on an issue. They say, "ok" and I say what I think about given legislation. If they aren't being rude, they get my name and address and I receive a written form letter back.
This is how I do it. I'm sure some of you look up the bill numbers and all of that. I figure they know what I'm talking about when I name a very specific piece of legislation ("the health care bill that recently passed out of House committee").
Your cell phone has no long distance charges, it takes about 2 minutes to do, so there's no excuse for not doing it. I always contact them regardless of their position or party.