You don't need to know the nature of the calls.
Being an intelligence asset to a foreign country is treason.
There's no situation where a presidential candidate (or his close advisors) are exchanging calls with the KGB that is *not* treason.
It doesn't matter if they were simply exchanging their opinions on the weather, you cannot communicate with foreign intelligence agencies without reporting it immediately. Failure to report makes you an intelligence asset, and HIGHLY susceptible to blackmail, which generally invalidates any chance of decent security clearance.
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Diplomacy is reported and in the open. Espionage is diplomacy done by other means.
And for instance, are you suggesting that Russia fixing the election for Trump was simply "diplomacy" and not Treason ?