If you are gonna call me and expect to be called back, 1. Leave a message! If it's not important enough for you to try getting my attention, it's not important enough for me to chase you down and find out what you wanted. And 2, ACTUALLY SAY WHAT YOU WANT IN THE MESSAGE. My doctors are especially bad at this for some reason, leaving some generic message that just says to return their call, which leads me to cluelessly try to navigate the robot menu until I find a person, only to be asked what I'm calling to ask about and not even being able to answer. When I call them, I get instructed to leave my name, birth date, phone number, and reason for calling. Surely it's not so hard for them to give me at least some of that in return so I know what the hell I'm calling back for, or who I'm supposed to even be asking for when I do. Jeez.
In conclusion, if I don't know what you want and it doesn't look like it was important, don't be surprised if I ignore that missed call.