Currently, it is not possible to connect a fax machine to an Analog Telephone Adaptor (ATA) that is connected to Cisco CallManager Express through Skinny Client Control Protocol (SCCP). Fax passthrough on Cisco CallManager Express or ATA skinny code has not been tested and is not a supported solution.
The feature request for this functionality is being tracked by Cisco bug ID CSCed47764: Call Manager Express does not allow for fax passthrough configuration.
T.38 and Cisco Fax Relay support fax passthrough in an ATA or Cisco CallManager Express environment. To accomplish this, choose from one of these options: