Let's review the timeline:
July 13, 2019:
Trump flat-out tells ABC News that, if a foreign country offered dirt on a political rival, he would absolutely accept it.
July 18, 2019:
DoD announces plans to provide aid to Ukraine, including money for "sniper rifles, rocket-propelled grenade launchers, and counter-artillery radars"
July 25, 2019: Trump is known to make a call to the Ukrainian President. It's strongly believed this is the call in question, as such a call would 100% be monitored.
July 28, 2019: Dan Coats resigns or is fired.
Aug 3, 2019:
OMB writes a letter to Congress, saying that the aid is being put on hold. The letter is not Ukraine specific. The letter does not mention corruption or other countries. The letter is about funding.
Aug 8, 2019: Trump nominates Maguire.
Coats calls his deputy during a meeting and asked her to resign. She does.
Aug 12, 2019: Whistleblower reports to ICIG.
Aug 26, 2019: ICIG reports to Maguire.
Sept 2, 2019: A week passes. By law, Maguire blew the deadline, by not reporting the whisteblower to the House and Senate.
Sept 9, 2019: Two weeks have passed. ICIG goes to Congress directly.
Sept 10, 2019: Schiff writes to Maguire, demanding the whistleblower info.
Sept 11, 2019: Trump's hold on the Ukraine funds suddenly ends. Trump adds $141 million to it. Also Bolton resigns, which could possibly be related, but I don't think that's the case. Feel free to opine otherwise.
Sept 13, 2019: Schiff subpoenas Maguire.
Sept 16, 2019: Three weeks have passed. ICIG goes to Congress again.
Sept 18, 2019: WaPo breaks story.
Sept 19, 2019: Maguire talks to HIC behind closed doors. Schiff is unimpressed, saying Maguire did not give the relevant info, and that he believes the relevant info is being intentionally withheld.