I don't mind helping most of the time. If I'm to busy I just say I'm sorry I don't know the number. I've asked people for help myself in the past...play it forward tends to be my motto.
What I don't like are the bill collectors who call our business line looking for someone who used to have this number. I've tried being nice, I've tried explaining they no longer have this number and to please take it off their list as it is a business, but they still keep calling. Now I just hang up.