No clues unless you provide more details on the modem, event logs and lastly
T30 Logs.
For now i can only suggest windows update.

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Shyama Hembram
Microsoft Printing, Imaging and Fax Team
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> When an incoming fax is being received, the process lasts
> exactly 15 seconds and the whole PC system freezes,
> causing me to shut down and restart, not by using
> Ctl+Alt+Delete, but by having to shut the power off and
> starting again. Sending faxes is all OK