Go to start>settings>control panel> and double click modems, select your modem and click properties.
Under the modem tab, adjust the modem slider or go to connection tab, click advanced button to bring up extra settings field.
Enter one of the following commands in extra settings,
ATM0= to silence modem completely
ATM1= all noises till a connection
ATM2= noise during use only
ATM3= no dialing sounds, noise while connection is established.
hope these help, good luck