Folks; I am attempting to deploy T.38 Fax (ZRTP Secure) with FreeSwitch / FusionPBX and am seeking system architectural advice regarding how to approach this. Question #1: Recommended T.38 capable ATA (and settings) for reliable FAX. Approach #1 ZRTP mitm with T.38 SIP Extension (either T.38 capable ATA or soft endpoint) - preferred approach T.38 SIP endpoint <-> FS#1 / mitm <-> cloud <-> FS#2 mitm <-> T.38 SIP endpoint I am using a soft T.38 SIP endpoint, plugin for Windows Fax and Scan from faxback, available here: http://www.faxback.com/Products/Trialware.aspx?NET_SAT_EDITION=Small%20Busin ess SIP Gateway, just prior to bridge: <action application="export" data="fax_enable_t38_request=true"/> <action application="set" data="execute_on_answer=t38_gateway peer"/> Extension, just prior to bridge: <action application="set" data="fax_enable_t38_request=true"/> <action application="set" data="execute_on_answer=t38_gateway self"/> What I am seeing (two FS systems), each with T.38 soft extensions, attempting to fax each other. ZRTP negotiation fails (Max retransmissions fail) -> NOZRTP ONE SDP is T38Fax (extension or gateway), Other is PCMU/8000 The extension of T.38 enters state [ready] and immediately hangs up, I assume from SDP mismatch Question #2: Anyone successfully done this, if so, how? Question #3: Is what I am attempting architecturally possible? Can T.38 be mitm'd? Question #4: If so, advice regarding how to debug the ZRTP negotiation failure and SDP negotiation failure. Approach #2: FS terminates incoming T.38 fax, stores and forwards to fax extension, FS terminates outgoing T.38 fax, stores and forwards to SIP gateway to final destination. T.38 SIP endpoint <-> FS#1 / terminate fax <-> FS#1 forward fax <-> cloud <-> FS#2 / terminate fax <-> FS#2 forward fax <-> T.38 SIP endpoint Question #5: Will this work? Anyone done it, how? ZRTP gotcha's? Thanks; Bill