Answer tone

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Answer tone is a feature of wireline modems.

The answer tone is the first signal sent by the answering modem after the billing delay. In its most basic form, it is a single continuous tone with a frequency of 2100 Hz (or 2225 Hz for Bell modes).

The plain 2100 Hz tone is meant to disable echo suppressors on international trunk connections. It may include 180° phase reversals at intervals of 450 ms to disable network echo cancellers. It may also be modulated by a low frequency signal to indicate ITU-T V.8 capability.

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