I have recently set up a computer for a friend. Previously she had an
older machine with a 56K modem which worked without problems.
I replaced it with a new machine (PC with Windows XP) with a new built-in
modem. When the modem is plugged in (whether switched on or not)the volume
on the telephone is very much reduced.
I have checked all of the wiring from the BT connection point, and it is
OK. There are no pops or crackles on the line, and the connection doesn't
go down, but the maximum speed that the modem manages is only 28K.
The house is very close to the sea, and it is possible that that the
external phone wiring is corroded. For that reason I would have suspected
that wiring as the culprit, but the line-test facility and the quiet line
tests show no problems, and the previous modem (bought 8 years ago) caused
no troubles, so I now suspect the modem. I would have thought that a newer
modem would have imposed a smaller rather than a larger load, but perhaps
this isn't the case.