First, let me start by saying I know how busy everyone in radio is and sometimes it's hard to respond to people. But it's not impossible.
I have a rule that I will call or e-mail everyone back within 24 business hours (unless I'm traveling, then I put a message on my voice mail). Is it tough? Absolutely. Is it worth it? Absolutely. I consider it part of my job and I always budget time into my daily to do list to get this task done.
Nothing is more frustrating than someone who doesn't call me back or is non responsive to an e-mail. Remember the Golden Rule..."One should treat others as one would like others to treat oneself."
Make calling everyone back part of your daily duties. And if you are in the same office, get up and talk to them face to face.
It's just good business.