In Knives Out (2019), Detective Benoit reveals Ransom's plan:
Benoit: Marta will get arrested for killing Fran and Harlan. And it would have worked if we hadn't have brought you in for questioning so you could not make your anonymous call. And if Fran had not stashed a safety copy of the tox report.
Why couldn't he make an anonymous call if the police haven't brought him in for questioning?