1A service that can fax a document automatically on request, in response to the fax number (and sometimes other details) being entered into a database by telephone or email; a message or document faxed in this way.
A trademark in the U.S.
2A questionnaire or similar document inviting a response by fax; such a response.
1980s; earliest use found in PR Newswire. From fax + back, after callback.