Easy to find when someone calls
I have a bright red Waiting For Support Folder in my desk drawer. I very frequently call someone about a particular issue and have to leave a voice message. When they call back I am always in the middle of something else so I can maximize this interruption by grabbing the folder as we say hello and finding the paper easily without having to interrupt the call.
The timing of action for waiting fors is out of my control, so I don't like putting them in the tickler unless it is a specific date I'm waiting for, rather than somebody else.