Hi, I would like to setup FS1 as a 'ZRTP gateway' to a none-zrtp sip client. Here is my net setup: Client1 (without ZRTP)-> LAN -> FS1 (Gateway: register to FS2) -> NAT -> Internet-> Public IP (not NAT) FS2 -> CSipSimple (ZRTP enabled). Client1: Ø Ex#1000, Registered over LAN to FS1. FS1: Ø Being NAT with dynamic IP address Ø Registered as a Gateway to FS2 (in Internal profile). So FS1 is extended on FS2. Ø Setup as 'Proxy-Media = false' && zrtp_enrollment=true (trusted MITM). FS2: Ø Connected with static public IP address (not behind NAT) Ø Setup as 'Proxy-Media = true' && inbound-late-negotiation=true. CSipSimple: Ø Behind NAT (remote WiFi/3G) and is registered on FS2 (Internal profile). FS2 is successfully serving multiple CSipSimple ZRTP clients with end-to-end ZRTP secure calls. The problem: When calling from Client1/FS1 to FS2/CSipSimple, then FS1 shows ' WARNING! Incoming ZRTP CRC validation fails' and FS2 shows ' ZRTP not negotiated on both sides; disabling ZRTP passthr '. I have tested FS1 locally with CSipSimple and it manages to connect with ZRTP/MITM so it is capable of ZRTP. I follow up what have been recommended on the forum to extend FS1 and to verify matching codecs (I forced PCMU only on all devices) and to check RTP/UDP ports flow (SIP trace). I would appreciate any help on how to setup end-to-end ZRTP calls between FS1 and remote CsipSimple (FS2). Regards Assaf