On Wed 14 Aug at 21:44 Malcolm Young (3:800/809.1701) wrote to Arthur Marsh:

 MY> Does this mean that if we go back to 300 bps modems this 
 MY> method of detecting data calls will not work ?

Well, V.21 at 300 bps (which has a pure 2100 Hz answer tone) is used by
facsimile machines which I don't believe are the target of this Telstra
exercise.

 MY> Can an old ARE exchange detect data calls?

No, but in some transit exchange there is probably some logic to determine
whether a long distance domestic call needs echo-cancellation, and if so,
that transit exchange could detect an echo-cancelling data call, which uses
an answer tone of 2100 Hz with phase reversals.

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