If the modem is damaged then, depending on how it was damaged, it may cause a problem with the dialy call over then network. Even though the daily call is configured to go out over the network interface the tivo still trys to make the modem hang up after the call is complete.

The fix is to telnet into the tivo and to type the following:

mount -o remount,rw /
mv /dev/cua1 /dev/cua1.old
mount -o remount,ro /