the following link explian how trapcall.com work.

http://askville.amazon.com/Trap-Call-work-unmask-blocked-caller-ID-numbers/AnswerViewer.do?requestId=40144836

And

I have had a investigation with the trapcall, I found it have some limitation to numbers, as I have test two number with me, all is said not supported. I think it maybe have coopration with the carrier.

As I think it need redirect the incomming call to the toll-free number and get its callerid, so if without a mobile app, it need the user to setup the redirect by themself, if with a mobile app, the redirect filter can be done by the mobile app.