I have an rpm that I just click on the disable modem button and it dissconnects, and then when I need to go back on I just hit the enable modem button works great, i cannt remember where I downloaded it from though, the name of it is :
e-smith-dialup-0.1-1.noarch.rpm
If you cannt find it I can email it to you directly, it is an awesome rpm for users of 56k modems

It allows you to change your dial-up settings (username/pass/phone number), phone line settings (tone/wait for tone), modem settings (enable/disable modem speaker/ modem serial port/ serial port speed) and dial-up policy (how often it wiil ring)
Cyrus Bharda