Don't tell the call spammers this or they'll train all their "agents" to start phone calls with "what's up, bro" or something they think is the stereotypical opposite of formal.
"Am I speaking with X? This is Y from Z Corp." is okay.
"Am I speaking with X?..." is a spammer, a complaint, or someone trying to serve me papers.
(in the US)
Nearly all calls i get go to voicemail by default, it's been a great filter with its voice transcription!
They want to make sure they called the right person. Except they know everyone hates getting called like this, so they take "who is this?" as affirmation and then proceed to tell you their company and the call.