Well that's just it: I had IP addresses and public posts from the perpetrators and NCMEC never called me back.
Also this NYT story: https://www.nytimes.com/interactive/2019/09/28/us/child-sex-...
IWF is the British hotline.
They exchange information with each other and pass information through to law enforcement after reviewing it.
Different hotlines may differ in what reports they're willing to take depending on what's banned locally. IWF is legally mandated to take reports on cartoons. NCMEC isn't in theory but they might take it and pass it onto the user's country due to lack of manpower.
As a comparison, CERT operate the same model: informing them of an attack can lead to them becoming bound into LEA activity which requires them not to disclose.