a call can also be defined as an announcement. A common casual usage for this would be "Last call for alcohol!" at a bar. This is the bar tender telling his patrons he is about to stop selling alcohol and they should buy their last drink now. In this case, depending on the context, these could all be both requirements and announcements. It's common to "make a call" for something to ensure that the requirement has been met. For instance the "check in call" could be a requirement for everyone to check in, and the announcement that they must do so. These are not phone calls (unless, of course, the announcement is being made by telephone. But that would be simply coincidence, and would not change the meaning of the word "call" in this context.)