Common courtesy is disappearing.
When my phone rings and I pick it up, I expect the person who is calling me to respond when I answer - but these days they so often don't.
I do not expect to have to keep saying: "Hello............................ hello..................... hello......................." every time, before the thoughtless people who are ringing me take their time to pick up their own handsets to get ready to speak to me.
Their bad manners deserve a bad mannered response.
So, speak immediately - or I will assume you are a scammer and hang up.