For the most part you'll just get a form letter with little to no information in it. It will also almost certainly be written by a staff member that has little to no experience with the policy matter in question.

Responding to a bunch of people individually doesn't scale, so that's not terribly unreasonable. Still, if you call, it does a couple of things 1) you do find out what they think about the issue 2) they know that their constituents care about the issue.