Defining a dialog box field as "mandatory" only works for some types of fields. For example, it works for a text field, but it does not work for other types, like date.
Use "savecall." See http://docs.day.com/en/home/communique_3/docucq355/programming/libraries2/cfclibrary/cfccontrols18/cfccontrols0/cfccontrols2.html for details.
Currently, "mandatory field" applies to text fields only.