First of all, edit the file /usr/local/freeswitch/conf/autoload_config/switch.conf.xml and set rtp-enable-zrtp to "true":

<param name="rtp-enable-zrtp" value="true"/> 

Then you can enable or disable ZRTP support from within your Dialplan using the following command:

<action application="set" data="zrtp_secure_media=[true|false]"/> 

When ZRTP is being negotiated, you will see the following line on the FreeSWITCH console indicating ZRTP is being offered:

[DEBUG] switch_rtp.c:928 [ zrtp main]: START SESSION INITIALIZATION.

The ZRTP protocol will then begin injecting ZRTP negotiation packets into the RTP stream. If ZRTP is successfully started for a session, you will see a series of ZRTP log messages followed by a confirmation message that the channel is now secure, such as the following:

[zrtp protoco]: Enter state SECURE (DH).

You will also see that a cache of the selected shared secret was auto-stored, which will be used for comparison purposes on the next call:

[ zrtp cache] Storing ZRTP cache to </usr/local/freeswitch/db/zrtp.dat>...