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Example of a North American Off-hook tone

Listen to an off-hook tone from North America.

Some central office switches in the United States, notably older GTD-5 EAX systems, utilize a single frequency tone, 480 Hz, known as High Tone for this purpose. Listen to an older 480Hz off-hook tone.

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The off-hook tone (also off-hook warning, howling tone, or howler tone) is a telephony signal for alerting a user that the telephone has been left off-hook without use for an extended period, effectively disabling the telephone line. (Full article...)