Its not a JB app or even an app, its service you can install on any cellphone in the US.
Its called trapcall.
When someone calls you with a blocked number you reject the call, the call is forwared to that companys 800 number and then back to your cellphone with the true called ID revealed.
It used to be free when it first came out, now its like $5 per month.
http://www.trapcall.com/