My take, for what it is worth!

I believe the key to all these perhaps lie in Fredo's boathouse outburst
Michael certainly believed until Fredo's outburst, Roth mislead Fredo and he didn't know they were planing to kill Michael

My view and I hope no sleeping with the fishes for me! among others,

• Fredo undoubtedly wanted to be Don Absolutely no question about that
• However 'ostrich' Fredo may have closed his mind as to how he is getting the Donship obviously unless Michael, the current Don is dead
• After the Tahoe shooting no more self-deluded beguile excuse as to how Fredo was getting the Donship
• Fredo arguably opened the window drapes thus 'identifying' Michael's bedroom and knew it was gonna be a hit
• Fredo tells Michael “I didn't know it was gonna be a hit Mike I swear to god I didn't know it was going to be a hit” but Fredo is never going to admit that he set his kid brother up to be murdered
• Fredo's “You lied to me” to Ola could mean that they promised Michael would have been dead and Fredo would have been the new Don by now, the something in it for Fredo
• Even Fredo would have known there was no way he could operate outside the Corleone business, something on his own
• Ola's “Your brother's not going to find out we talked” could be a veiled threat, or else
• Fredo's “I don't know what you're talking about” could be, leave me alone “you got me in deep enough already”
• We are none the wiser as to what exactly was the little help Fredo gave Ola but whatever it was it was undoubtedly instrumental in the Tahoe shooting and nothing to do with closing the deal fast! .
• If Fredo did not want to harm Michael why did Fredo still withhold information from Michael like "The Senate lawyer Questadt belongs to Roth"
• Fredo's continued silence was because Fredo still wanted to harm Michael and try to send him to Prison for Don Fredo
• How hard it was, like pulling teeth, for Michael to extract the information from Fredo in the boathouse

The thorn! in my theory is why did Ola phone Fredo about Pentangeli's meeting with the Rosato brothers?
Roth and Ola would have known, always knew that Fredo has no involvement in the family business and would know nothing about the meeting

So, what was the significance of Ola's late night phone call to Fredo?
The only reason I can think of for the phone call scene is to convey the information to the prospective audience that Fredo was the traitor
Amen to that!