[Freeswitch-users] Sdp manipulation help

I am having trouble putting srtp to optional mode. UAs send sdp with rtp/avp and a:crypto lines which im permitting. But i want to decide whether the call uses srtp or zrtp. The UAs send also zrtp-hash. I export sip_secure_media to false but am still seeing the a:crypto lines being sent to the b-leg. I cut them off in my dialplan with set switch_r_sdp, but that breaks the whole negotiation, because the sdp isn't properly patched afterwards for nat( i also set a bunch of other vars afterwards). Is there a polite way of telling fs to remove the a:crypto lines and break the srtp and thus activating zrtp. Running 1.2.8git with inbound_zrtp_passthru to true.