How can you find out why the cops tapped your frinds cell phone?

Ok first off if the police tapped your friends cell phone and you know about it chances are your gonna already In the conversation the cops been having. In order for law enforcement to tap a cell phone they need a warrant in which case a judge needs to sign off on and in order for this to happen the officers or agents must provide hard factual evidence for this whole procces to take place. So if the cops got your it's cell on lock sorry to say but he's in a lot of trouble