Someplace you've got the option "Wake on Ring" turned on.
Where the option will be will depend on whether the modem
is built into the motherboard, or is a seperate card. If
it's a separate card, you should find it in the modem
configuration. If it's built into the motherboard, it may
be in the BIOS configuration. The feature is there so
people can have their computer wake up when the phone
rings to receive a fax, take a voice message, etc.
If you can't find it in the configuration, and the modem
is a separate card. see if there is a little cable going
from the modem card to the motherboard. Removing the
cable may stop it.
Marty