How to cancel a call from spam to normal list?

My mobile phone is Samsung Galaxy Mega 5.8 (GTi9152). A few minutes ago I received a call from a number which is not my contact list. Meanehile, I was sending an urgent message to an important person in Facebook. The call prevented my task. So I rejected the call. But by mistake I added that number to spam when I got an option to fo so. Now how can I remove it from spam list to normal.