Modem cuts out phone

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.
Any thoughts?
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Try swapping the 2 wires (ring and tip) on the phone jack. Some devices are not affected by the swapping of these 2 line, however some others can have wierd effects...

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If you unplug the modem, does the volume uncrease on the phone? If so, it has to be a bad modem.
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Back in the days didn't we need to worry about something called the 'REN' Ringer Equivalency Number IIRC. Each device has a REN and there is a maximum REN total per line in the house IIRC. I have no idea what the maximum REN is but it would be interesting to see if the REN# on the new modem is higher than the old modem.
What happens if you leave the modem and unplug all phones, answering machines, etc but one. Is the volume normal on the one phone?
Just a thought.
-B

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Yes, unplugging the modem brings the volume back. Seems I need to complain to the modem supplier - which I suspected. Thanks for confirming this.
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Integrated Modems = Junk. They are usually soft modems which are processor dependent and are not as fast nor efficient as hardware version.
Unless you are connecting to an ISP that supports 56K, 28.8K seems about right. The max speed a phone line support is 33.6K, but rarely doesn't a modem connect at that rate (line quality)
If it was working before the upgrade, then it's the el cheapo integrated modem. You can either disable it and install a new hardware modem ($50.00 or so) or install the older modem into the PC (if it's a PCI Modem).

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