They answered this in the movie when Tom said, "Roth played this one beautifully." Roth engineered the attempted hit on Frank Pentangelli, but made it look like it was Michael. Remember, in an earlier meeting, Michael asked Roth for permission to kill Pentangelli. So Roth instructed the button men to say "Michael Corleone says hello" when they attempted to kill Frankie. Even though the attempt to kill was unsuccessful, Roth fooled Frankie into thinking that Michael tried to have him killed. This triggered Frankie testifying against Michael to the Senate Committee.