How do i figure out whos calling me if thier numbers private?

ok i keep geting calls from a private number and when ever i answer itll be silent for a couple of secs then they hang up. how do i figure out thier number to call them back?How do i figure out whos calling me if thier numbers private?
cant...





just block private callers *77





edit:


with cell phones you can use trapcall.comHow do i figure out whos calling me if thier numbers private?
some phone companies allow the use of *67 - right after the call, punch the *67 code, and it should tell you the number. You can check with your local phone company to see what their code is...