Andreas:
 > As it seems, line and phone connectors are connected
 > "in parallel" i.e. pins a-a1 b-b1 are _not_
 > disconnected in off hook state...
 > 
 > Is there a way out ? For example an electronic patch or
 > even a software switch ???
There is no way to change the modem behavior.  Places such
as Radio Shack in the U.S. have a device called a Teleprotector
which prevents a phone device from using the line if the line
is already being used.  Such a device may resolve the problem.
 Mark.
 Supra Tech Support.
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