The truth is my job as an intern, as was the job of all other interns that I met while in DC, was to take constituent calls and also open constituent mail. However, no information was ever actually relayed to the congressmen. We had a formatted response to each and every issue that the House could possibly vote on. Everything from internet poker, to any issue you could imagine. We would print out (and alter if necessary) the response to tailor it to the individual that called, emailed, or wrote a physical letter. The congressman's signature was stamped at the bottom of the letter and sent back to the constituent, giving the allusion of due diligence on the congressman's part.
I was extremely surprised and disappointed at the same time at how commonplace this was. Pretty much every intern I asked about it went through the same drill. It's just another thing about our government and "representative democracy" that really irked me. So whenever I see ads urging people to call or write their congressman, I think back to this and realize further how powerless we really are.
The best way to exert influence over your congressman is to donate lots of money and become a memorable name that can get in contact with the actual representative him/herself. Hell, that's how I got the internship. This is one of the reasons I sympathize with the OWS movement.