Hi; Faxback Support - SUP-00019595 I am attempting to get T38 Fax working between two FS/FusionPBX installations using faxback VOIP plugin extensions. The fax TX (FS#1) endpoint drops the call due to "Ack Timeout" after two pages. Faxback VoIP client plugin: http://www.faxback.com/Products/FreeMSFaxDownload.aspx I am able to successfully receive a fax using this faxback endpoint if transmitted by FS/spandsp. Note that zrtp is disabled for fax calls, due to T38 udpl (not rtp media) incompatibility Thanks Brian West!: "T.38 Fax takes place over UDPTL, There for it can NOT be encrypted as there was never really a standard for doing so." Details (both FS's): SIP Provider: freephoneline.ca (claims T38 capable) freeswitch - 1.4.18 x86_64 Topology: FS#1: rossco.org, using faxback plugin (x1019) to send fax FS#2:rossco.duckdns.org, using faxback plugin (x1019) to receive fax (faxback x1019)FS#1 -> cloud -> FS#2(faxback x1019) Key variables for fax sender (FS#1), during call: $domain= rossco.org Faxback Fax extension: 1019 t38_passthru = true zrtp_secure_media = false fax_enable_t38 = false (not set) fax_enable_t38_request = false (not set) fax_use_ecm = off (not set) inbound-late-negotiation = true ignore_early_media = true Key variables for fax receiver (FS#2), during call: $domain= rossco.duckdns.org Faxback Fax extension: 1019 t38_passthru = true zrtp_secure_media = false fax_enable_t38 = false (not set) fax_enable_t38_request = false (not set) fax_use_ecm = false (not set) inbound-late-negotiation = true ignore_early_media = false Full Logs w/siptrace: Sender(rossco.org): https://pastebin.freeswitch.org/24117 Receiver(rossco.duckdns.org): https://pastebin.freeswitch.org/24118 Note that the SDP's are correct for T38: v=0 o=Sippy 177245968 2 IN IP4 208.65.240.165 s=FreeSWITCH t=0 0 m=image 49714 udptl t38 c=IN IP4 208.65.240.165 a=T38FaxVersion:0 a=T38MaxBitRate:14400 a=T38FaxRateManagement:transferredTCF a=T38FaxMaxBuffer:200 a=T38FaxMaxDatagram:72 a=T38FaxUdpEC:t38UDPRedundancy Questions: 1 - According to my interpretation of logs, T38 and t38_passthru is negotiated 2 - I do not have the expertise (yet) to determine the reason for "Ack Timeout" 2 - What am I missing? Thanks; Bill Ross