You can always just accept the call and wait for the other party to speak. If it's a machine-assisted call, silence doesn't trigger the answer detector, and it

You can always just accept the call and wait for the other party to speak. If it's a machine-assisted call, silence doesn't trigger the answer detector, and it hangs up after a few seconds, possibly also marking your number as no-contact. A human caller will eventually ask if anyone is listening. They called you, after all. They can just say what they wanted to say whether you acknowledge them or not.