Telephony - Signals from modems to PSTN

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Telephone line signalling

Calling tone

The calling tone is sent by the calling modem to indicate that a data or fax call is being placed.

It is a slow period tone, where the On period is shorter than the Off period.

For data, it is nominally 1300 Hz at 0.6/1.75 (on/off in seconds).

For fax, it is nominally 1100 Hz at 0.5/3.0 (on/off in seconds).