[nostrademons]

They aggregate the responses and tally them up statistically. The assumption is that for every person who bothers to call in, 10+ people, perhaps even hundreds, feel the same way. (I assume they'd model based on the relative number of total calls across all issues vs. voters in the district). Then this becomes data for the representative to know how their constituents feel about the issue.

The rep isn't going to listen to you specifically - you get a form letter back (sometimes years later), and the only thing that matters is the issue and whether you're yay or nay. But because so few people call in, it makes your opinion (on that issue, at least) weight much more highly than the average voter.