| Telephone line signalling |
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| Answer tone
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 disables echo suppressors on international trunk connections.
It may include 180° phase reversals at intervals of 450±25 ms to disable network echo cancellers.
It may also be 20% amplitude modulated by a 15 Hz sinewave to indicate ITU-T V.8 capability.
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