Hi, I have two FS's (FS1, FS2) with ZRTP enable in two sites. I can make end-to-end ZRTP calls between two CSipSimple/ZRTP clients over the two FS's bridge where both FS's are set to 'proxy_media=true'. CSipSimple -> FS1 ---Internet -à FS2 -> CSipSimple. Now I connected a PBX to each FS with SIP-Trunk. PBX -> FS1 ------- FS2 -> PBX. When calling locally from CSipSimple to a PBX extension on the same FS then the ZRTP is terminated by the FS as MITM which is OK (FS dialplan 'proxy_media=false'). What I cannot figure out is how to terminate ZRTP call between CSipSimple/FS1 and FS2/PBX.ext and vice versa. CSipSimple -> FS1 ------- FS2 -> PBX.ext. I've tried to setup: FS1 with 'proxy_media=true' (passthrough ZRTP) + FS2 with 'proxy_media=false' (terminate ZRTP). OR FS1 and FS2 with 'proxy_media=false' . All results with ' ZRTP not negotiated on both sides; disabling ZRTP passthru mode'. Any help will be appreciated. Regards Assaf