Another student used my completed work, what should I do? I took a class a year ago, another student currently taking the same class had asked about my assignments to guide her in her work. I explained to her how intense the class is and sent her a copy of my work so she sees how much efforts she needs to put on that paper.

Without my authorization, she used my work for her assignment and now we are both in a mess. The school had contacted me about this matter and wants me to call them for a phone conference about this matter. The school policy includes but not limited to "no sharing of old assignment" or else disciplinary action will be taken. So how can one approach this case when asked to report by telephone?
If I understand correctly, the other student took the class after you took it, so it's clear that she cheated from you, not vice versa. I suspect they merely want to talk to you to find out exactly what happened. You are probably not in any trouble, unless there is a school policy against sharing old assignments, or the professor asked students not to do this, or you knew that she would have the same assignment when she took the class. So I would simply answer any questions they have, and explain why you did what you did.