On the track of Zone Alarm (the firewall): commented out references in autoexec.bat to tvdumpflags, because I was seeing an unusual number of freezes while trying to download/play a large Quicktime file, and also while running the modem for a while.
3/22/2006: uninstalled old driver, and re-installed latest version of modem driver with call waiting app. Am about to test it. First test seems promising. Called using a friend's phone, had 15 seconds to answer call, or ignore, answered it, talked for a few seconds to verify voice connection, and then hung up, and was automatically switched back.
3/24: Success in a 6 hour download! Cranked back the receive modem buffer, and turned off all software in the system tray that didn't have anything to do with the download.
3/27: Installed update of firewall. On the trail of sporatic crashes trying to download update (10 meg). Cannot find dialog box to update modem buffers.