Is this auto-detecting fax tones in his voicemail? I don't think that I have ever setup a Stratagy like that. Generally, I use a separate phone number for faxes to a separate voicemail box.

You can still use set faxes to go to a different voicemail box. You can have the faxes get emailed and then deleted from the voicemail box. Are voicemail messages getting sent by email also? Typically I will set the Stratagy to delete on send unless the user specifically wants to delete them in 2 places.