Read this before blaming the dog
Your phone speaker may not play the tone. Above ~18 kHz many phone and laptop speakers output almost nothing, at any volume. The button works, the waveform is correct, and the room stays silent. Before a training session, run the speaker reality check on the main generator — it uses your mic to verify which frequencies this device actually produces. If 18 kHz fails, train at 15–16 kHz instead; dogs hear those fine too.
A whistle is a marker, not a command. Dogs don't arrive pre-programmed to care about 18 kHz. Pair the tone with a treat consistently — tone, treat, tone, treat — and it becomes a recall signal. Without the pairing it's just a noise that briefly rotates one ear.
It won't stop a stranger's barking dog. Sites promising an "anti-barking frequency" are selling you a placebo with a slider. A novel sound may interrupt barking once or twice; habituation follows within days. Training changes behavior, frequencies don't.
Why you can't hear it (and your dog can)
Human hearing tops out around 17–20 kHz in childhood and slides down from there. Dogs keep going to roughly 45 kHz. A tone at 18–20 kHz sits in the gap: comfortably inside a dog's range, at or past the edge of yours. If you're under 25 you'll probably hear this whistle faintly; over 40, likely nothing — check your own ceiling with the hearing range test.