It shouldn't matter who the person is. What matters is whether they can prove what he's saying.
If it turns out to all be false then they can lose their anonymity and whistleblower protections. But in the meantime we don't have to know.
Oh but we DO have to know. Since the person who actually 'heard' the conversation was someone else, who also doesn't want their identity reveled, and passed the 'info' to the 'whistleblower' (who isn't really a whistleblower), the 'whistleblower' doesn't have any idea what was said and can't 'prove' a damn thing.
On the other hand, the president released the call transcripts, although he didn't have to. Thats all the proof you're gonna get, and all we need.
I cannot wait to find out who the players are - its gonna be epic.