On Fri 16 Aug at 10:17 Peter Collis (3:713/914.16) wrote to Arthur Marsh:

 AM> It has appeared in the media, but people haven't 
 AM> understood that an S12 exchange, having in-built 
 AM> echo-cancellers can detect the 2100 Hz tone with phase 
 AM> reversals of an echo-cancelling modem 
 AM> (V.32/V.32bis/V.34) answering a call and pass this 
 AM> information on to measuring/billing software by means of 
 AM> a software upgrade
 AM> only.

 PC> Does HST have these 2100hz tones if not it may be a way 
 PC> around the time charges.

If it were a HST-only modem, it would probably answer with 2100 Hz
*without* phase reversals (or possibly even the Bell 2225 Hz answer tone,
which I encountered once when calling South Africa).

If it supports V.32/V.32bis/V.34 it would answer with 2100 Hz incorporating
phase reversals which would be detected by the echo cancellers.

--- msgedsq 2.1