Unfortunately I can't find a source for this but I saw someone saying recently (and it makes sense) that a lot of congress fax numbers just convert the fax to an email message rather than physically printing it out.

What's a fax? ;-)

Seriously though, how do you know this?

It's not correct. It's easy to ignore faxes, and throw them away by the bucket-full without ever reading them. A phone call requires spending some low level staffer's time to listen to you. Getting inundated day-in, day-out by callers, all on one side of an issue, does have some effect on a congressperson's staff, and sometimes, the congressperson's actions.