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%20Business

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