I am sending fax from the Zoiper for testing and getting this error from server,
freeswitch version: FreeSWITCH Version 1.6.12~64bit ( 64bit)
OS:Debian GNU/Linux 8

https://pastebin.freeswitch.org/view/3cd3c224

2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:496 ==============================================================================
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:508 Fax processing not successful - result (35) Unexpected DCN while waiting for DCS or DIS.
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:513 Remote station id: 
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:514 Local station id:  SpanDSP Fax Ident
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:515 Pages transferred: 0
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:517 Total fax pages:   0
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:518 Image resolution:  0x0
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:519 Transfer Rate:     14400
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:521 ECM status         off
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:522 remote country:   
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:523 remote vendor:    
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:524 remote model:     
2016-12-14 12:42:12.795416 [DEBUG] mod_spandsp_fax.c:526 ==============================================================================