Trump said nothing during the call that reflects badly on him.
Stuttering John did falsely claimed to be another person (Bob Menendez).
Is it legal to make a phone call and claim to be another person? Or only if it’s in conjunction with a “real” crime, e.g, some kind of fraud? I don’t know.
But nothing else happened here except for Stuttering John making a phone call under a false identity.
Maybe the Secret Service just wants to know how he got the phone number.
Recording a call without advance permission is a serious crime in some states. Don’t know if that’s in play but it’s a serious felony in places.