If true, and not BS to sway the elections, even leaking that this happens and that we know that is happens is a serious national security issue. And no, it's not "whistleblowing"; Trump isn't running for election this cycle. And I remember when Obama wouldn't give up his iPhone, and insisted that his presidential daily brief be delivered by a custom-built iPad app.
And don't forget that the NSA and FBI have also been evesdropping on Trump in exactly the same way since before the election. Why should he take their warnings about China more seriously?
Oh, so two of them have been secured? It's just the one that isn't? I rather imagine the President can tell the difference between those three phones, and when he should be using a SCIF instead. So how is this anything other than an unauthorized illegal leak of damaging national security information touching on the sources and methods the FBI is usually so keen to protect?
Look, I'm not saying this is necessarily a good decision by Trump, if it is even true, but leaking it to harm Trump does nothing to improve the situation.
Here's another guess. Rosenstein refused to testify to Congress as planned because he learned he was going to be asked about this, and delayed the testimony so he could leak this and other "preparations" ahead of time.