Given the Trump/Sondland call was done on private cellphones (thus violating the presidential records act), I would guess there are two options for investigators....a
Get a warrant to seize Trump's cellphone, which if it's like most cellphones, at least logs all incoming calls, showing the number from which it originated.
Ask Russian intelligence if they would give us a copy of the recording they likely made in Kyiv.
Since these options have zero probability of working, I guess it's worth asking if some US branch of the intelligence services might have somehow made a record of the call.
But then, all Trump's paranoia and Deep State rant would be gloriously validated.
Is there some legal loophole for US intelligence?