Do airliners have horns for use on the ground?
(Source) Ground call horn.

There's a horn the pilots can sound to attract the attention of the ground engineer, so they can talk on the intercom (by wearing a headset). It's not loud enough to have any effect in the air—or when the engines are running loudly.

It's more of an annoying alarm really.

Taxi lights can be flashed on/off to attract the attention of ground crew, also opening a window and waving would work.

(Source)

(Source) Ground crew member wearing a headset connected to the plane.