Will Hargrave wrote in a message to Neil Croft:
 NC> Voices use a narrow but different band of frequencies to 
 NC> modems. All Teleco's use filtering to allow multiple calls to 
 NC> share single paths. Unfortunately, if the bandpass is set for 
 NC> voice then modems get clipped. You still only get the same 
 NC> bandwidth, just higher up the scale.
 WH> The whole point of modems is that the data is within the normal 
 WH> voice bandwidth. If they decide to set it differently, IMO the 
 WH> resultant service is not worthy of being called a phone line...
In the U.S., there is a distinction, and sometimes the customer sees it 
directly.  For example, with ISDN, there is a specific mode "speech" which is 
distinct from the mode "3.1 kHz audio."  The idea is that "speech" can be 
mangled up by compression or phase delay if convenient for the telephone 
company, while "3.1 kHz audio" should be carried with more respect.
Some local telephone companies actually ingnore the distinction and carry all 
analog calls identically, but others use and preserve the distinction.  You 
get to see this if you receive an incoming call on an ISDN line which is 
originated from a POTS analog line: since the caller has no way to control 
this, the telephone company sticks the mode flag on the call before making 
the ISDN call appearance.  What is seen at the ISDN side is a result of 
telephone company administrative policy.
 
-- Mike
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