We don’t document everything we don’t support, if we supported ZRTP it would be documented as being supported and it would appear in configuration somewhere. From the code itself, though:
ast_log(LOG_DEBUG, "Found Zfone extension in RTP stream - zrtp - not supported.\n");
Sangoma are not going to answer that until development has already started, and code has appeared in the public repository They can’t answer for other members of the community. My guess, though, would be “no”.
I know of noone actively working on such a thing, but I can not speak for the community in certainty as Asterisk is an open source project - so as long as submissions are license compatible, we review and potentially accept anything.
Sangoma took over DIgium earlier this year, so Asterisk is owned by Sangoma.
I have found no evidence that Sangoma patch Asterisk, and I think a patch to support ZRTP would be a major patch, so I don’t see how FreePBX can support it, but not standard Asterisk.
FreeSwitch is not owned by Sangoma. It seems to be owned by a company trading as FreeSwitch.com, but, as this is a US site, the underlying owner doesn’t seem to be identified.
It has a radically different internal architecture from that of Asterisk.