Hi
ZRTP is not supported in Asterisk, However I successfully deployed it with Freeswitch, I you want I can setup a separate server with ZRTP. Or I can migrate your existing system to FreeSwitch + FusionPBX. please let me know you thoughts.
About me
I am from Pakistan having 13 year experience with CTI development and have solid command on telecommunication protocols, technologies and software like Asterisk, Freeswitch, OpenSIPs, FreePBX etc ... for details please visit my profile.
Regards