I'll try a general approach to answering this.
Feedback is caused when the sound put out
by the speakers and/or headphone is picked up by
the microphone, and the gain is sufficient to allow
a "loop" to be created that increases the sound
each time.  Usually, turning down the gain on one
"end" or the other will solve the problem.
If this is a voice modem, you may have to look for
sound controls there...
Other times, the microphone may be focusing
the sound because of something very close to it.
Cupping a hand around a mic to stop feed back
will generally cause it, or make it worse, etc.
Hope this helps.
                         Rick Glazier

----- Original Message -----
From: "Thanh Nguyen Duc" <[log in to unmask]>
I ' ve got  Sm56 motorola modem, when dial telephone,
always hear a feedback from microphone to headphone,
after few minutes, the computer was hang up.

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