Go for it.. Don't use your personal phone number, or propose that you call him if he can pay for it.
Wall Street Journal is like a newspaper mostly targeted in finances.. So my guess would be he just want's to interview you about your sources, warez in general and the article would be about the people downloading warez and the financial damage that does to some big companies.
Its a well reputed magazine, you could ask for a monetary reward, though i doubt you are the only one he contacted about this.