The Update failed : Unable to access SFTP server error message appears when Network Directory is chosen and a valid backup location and service account to use as credentials is defined.
Make sure it uses port 22 and try again.
Once everything is setup, secure shell into the Cisco CallManager command line interface and issue the file get tftp command.
It asks for this information:
It downloads the file to the root directory that is specified in the program for SFTP.
Refer to Disaster Recovery System Administration Guide Release 5.0(4) for more information.