The whistleblower's report is probably based on recordings that exist.

[Kablooie]

Since the whistleblower comes from intel, it's not likely that he was in the room when Trump was on the phone.
That leaves him finding out about Trump's conversations from sources after the fact.
The only source that would be provable and definitive would be recordings but the president's phone can't be bugged, that's illegal.

But...

The Ukrainian president's phone could be bugged by the U.S. and any calls made to him would be recorded by our intel.
So the president's conversation could have been recorded from the bug on the Ukrainian side.

A problem arises in that a bug on the Ukrainian president's phone is probably top secret so to reveal a recording would be a problem.
This may be the excuse they are using to keep the report locked away but since the Intelligence Committee members have top secret clearance there would be no valid excuse to keep it from them.

[mr_lebowski]

1. Wouldn't be illegal to bug Trump's phone if his cocky-ass said 'go ahead I've got nothing to hide!'

Which is a scenario that's entirely possible in my estimation.

[NCLefty]

2. I hope that if Trump tries to release part of it, the intel community leaks the whole thing.

Trump: "It's deep state fake news, folks."

[DeminPennswoods]

3. This is how Flynn was caught

His conversations weren't monitored, but the Russians he talked to were.

Even if the US doesn't have an intercept, it's pretty likely the Russians do, giving them even more "kompromat" on Trump. Trump has established himself as a pathological liar. Putin could release an edited audio or transcript. No one but Trump cultists would believe his denials.