You'll get a good reputation for getting things done if you acknowledge everyone's calls and e-mails. Your answers can be brief, as this one is. Here's how I replied when a colleague asked me to review a newsletter before publication.
From: William S. Frank [wsfrank@careerlab.com]
Sent: September 17, 20— 12:34 PM
To: e-mail address
Subject: RE: Please review 'Newsletter'
Joe,
Got it, reviewed it.
Good start, let's discuss.
I'm free after 4:15 today.
:B