So the question is who gave him the transcripts? And is that legal?
“Legal” as far as monitoring the call? Gets kind of iffy, since intelligence would be monitoring calls received or made by Putin. Doesn’t mean Trump’s calls were being monitored.
“Legal” as far as disseminating the contents of said call between Trump and Putin to congressional leader(s) through covert means? Probably not but someone no doubt could mount a reasonable defense and deflect.
“Legal” as far as McCain sharing information gained via knowledge of the conversation, with a fictitious leader of Ukraine? No.