On this machine if I go to cont pnl/modem I find a vol slider that dose not seem to have any effect on this modem.
If I go to mycomp/dialup networking there I find a vol slider that does control the vol for this modem.
If neither one of those work I seem to recall an extra setting you could add to the init string to shut off the noisy beast.
If you need it post back and I'll go search.
How do I turn off the modem dial up sound? I know there are two adjustments in this thing, but I can only find the modem volume control - and that doesn't mute the modem. I used to know where it was.

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