Just a note: the Johnny-Fredo conversation establishes Fredo as a traitor, it also establishes Ola (and presumably Roth) as the adversary. We don't really have a scene like Luca's murder or the Sollozzo-Tom dialogue, so without Johnny's phone call the audience could go deep into GF2 uncertain who the bad guy is.
You are correct Johnny's phone call establishes to the audience, Fredo is the traitor in the family
We already know by the time of the phone call, Roth-Ola are the adversaries and their linking to the Tahoe shooting
