I do the same thing, when I call... its too easy for the person to say " I didn't get the message" even when they did. This is the only way you can make sure to cover the callers ass.
I also photocopy the schedule for each employee, and then have them initial the master copy that sits at work. There is no way they can say they didn't know about a shift.
I would say don't take offense, I am sure the caller has been burned before and got in trouble for it.