Modem doesn't know line is already busy

If the modem is tried to be used when the phone line is already busy with a voice conversation, it locks up the Dial Up, so even the dial up dialog box won't display.  Only thing you can do is reboot the computer.    Previous machines/modems would at least throw up a message - No Dial Tone,  in these situations.  It's a common phenomenon with a big family!!  Help??!!