Cats have sensitive hearing and hate loud noises, so it makes a lot of sense that a quiet whistle would work better than loudly calling out. Things like “pspsps” and “ksksks” get their attention too because it’s not very loud but has a lot of high frequencies that they can hear well.
They seem to like when you talk to them in a calm voice. They might not understand sentences, but they do recognize sounds. They’ll look at you funny if you suddenly start speaking a different language.