[Zope] Sending Fax through MailHost

Hi, I'm trying to put a fax number into the "To:" field of the mailhost header and am not having much luck. Obviously the standard for this is to address the email to [FAX:18479131145] but when I do [FAX:1<dtml-var faxnum>] it just tells me that there are no recipients designated. Any ideas? Thanks, chris