Thanks for reply,         I checked log messages at callee (IPv6 client - Linphone) and result was little surprising for me.  This IPv6 client as Callee didnt receive INVITE messages (I thought that yes because of Wireshark at SIP server).   So it is my bad somewhere in my local network.   But it is little interesting, that in other direction (from IPv6 caller to IPv4 callee) all Reply messages are received at IPv6 client and RTP/UDP packets go in both directions. I checked Iptables and Firewall at all nodes, but nothing (they all are in the same network) and I dont know what filteres messages to IPv6 callee client.   btw. the proxy server in the middle is SIP Protocol gateway, which translates messages between IPv4 and IPv6 and packets at this node are OK.   thanks for help       Jakub Hrabovsky