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

How do we know that?

Michael told Pentangeli It was Hyman Roth that tried to have me killed. I know it was him.

Michael to Roth: "FRANK PENTANGELI came to my home and he asked my permission to get rid of the Rosato brothers. When I refused, he tried to have me killed."

He said both things to both men at different times. The Tahoe shooting brought out the Master Manipulator in Michael. First he practically reduced Tom to tears by telling hom "You're my brother." Then he practically brings Pentangeli to his knees by yelling at him, "...in my house,...in my bedrooom.. ," then asks Pentangeli to help him take his revenge. "Michael, anything..." (Frankie practically in tears). That's when Michael asks him to put his head in the lion's mouth by meeting with the Rosatps. Then he bullshits Roth by callng him "Mister Roth," then telling him he's a "great man," then asking his permission to whack Pentangeli. Great stuff, Mike! You'd have made a good Presidential candidate.

First - Roth to Michael: You're a wise and considerate young man

only after – Michael to Roth: And you're a great man, Mr. Roth
There's much I can learn from you [like how not to hire assassins who can't shoot fish in a barrel!]