When it fails again, shutdown and remove modem and install in another open pci slot. When it re-installs again, see what the results are. If possible see if any new updates/upgrades are availble for that model modem.
I have a new USR 56K PCI modem, and although the first couple of weeks it was working fine, now upon each morning that I start up the PC, the modeum is not working (I use modem query under properties). After I delete the modem and re-start the PC, and it is all re-detected, it seems find. What could this be from?