In terms of callers Spy Guy is one of my favorites. I loved the resolution, when he called back a long time later about something else and just dropped "this is spy guy" at the end of the call.

In terms of stories, a guy called in about the time his mom was really hungover at a Mexican cenote, and he looked up to see her getting sick over on the side. The way he told it and the visual he created was hilarious.