I agree with Tora that your calling policy is very generous! I was a mean bitch when I was teaching: They could see me at my office hours, make an appointment, or email me. Calling at any time (except to my office) was verbotten. I did that after dealing with *far* too many post-midnight calls when I was a history TA.

As for your feelings of guilt when a student asks you about something that won't be on the exam: Shouldn't he/she walk out of the class with a good understanding of the concept whether it's on the exam or not? No, the student may not need it right then, but I'd think it's still a good thing to be sure he/she really understands.