On Wed 21 Aug at 23:04 Poe Lim (3:623/630) wrote to Arthur Marsh:

 PL> I guess what Ian is probably wondering (and so am I) is if 
 PL> there are no S12s at either end, perhaps there is no easy 
 PL> way to implement 2100Hz tone detection for billing purposes. 
 PL> I guess it will also depend where billing is generated if 
 PL> it's not from the local exchange.

It's certainly practical to screen non-local calls for 2100 Hz with phase
reversals, but local calls not on an S12 would need extra equipment
convenient to the call path.

This is getting off-topic here...

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