The phrase itself doesn’t matter, they are just listening for some indication of human speech at the start of the call.

If nothing is said, then the system believes that it is a machine doing the same thing.

Provided, of course, that their spam system doesn’t just starts playing a recorded message anyway.

Iirc, that’s usually a sign that the robot caller has found a machine on the other end, since most people first respond with some sort of greeting right off the bat.