You need to look carefully at the architecture for this. To avoid raising the priviledge level of the ASP.NET process, write the FAX record to a queue table in your database. A Windows Service can poll this table and send out the fax as it can readily be given access to the fax hardware.
For a sample of such a queue called from a web site, please see the common data solution at: http://commondata.codeplex.com/
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