Say again?

I was on my way home and as I walked into the subway station I heard an announcement saying something about all services having resumed. I thought to myself that perhaps having a two hour discussion after class had perhaps been to my advantage when I heard, "...if you have not heard this announcement please call ..."

I was not sure how one could call if they never heard the announcement? You wouldn't know that you have not heard it, if you've never heard it!