I see there's a mergfefield for the practice fax number. ie [party:getFaxNumber(party:getPractice())]. 

With Customer details, under Contacts there are fields for adding Location Contact, Phone Contact, Email Contact and Fax Contact.

However, under Administration | Organisation | Practice (or Practice Location), I have options only for Location Contact, Phone Contact and Email Contact: ie no Fax Contact.  (I tried adding Fax to Contact Purpose and adding the fax number that way, but that doesn't work.)

Where do I add the Practice's fax number so that it's picked up by the above mergefield?