In the movie Scarface (1983) Tony survives an attempt on his life which he suspects was ordered by his employer Frank Lopez. Before confronting Lopez, he instructs one of his own men to call Lopez at a certain time and say the phrase "We f***ed up, he got away." But seeing Montana come to him Lopez already knew the hit was unsuccessful- what purpose did the phone call serve?
Tony knew he would be in Frank's office when the phone rang, and he could directly confront Frank about the content of the call based on his reaction. Because Tony already knew what would be said to Frank, the fact that Frank didn't react like, "Who is this and WTF are you talking about?" let Tony know for sure that Frank called for the hit. Until that point, Tony wasn't 100% sure if it was Frank or not.