Hi, My Kamailio box works very well for a year, in front of a Freeswitch box. Nowadays, some of my users live problems about receiving calls. But this problem occurs on a couple of specific users. %1 of users complain about it. When I capture the traffic, I see that, Kamailio receives an invite package from freeswitch but takes no action. Normally it should response with a 100 trying message. Here is the INVITE package. 1.2.3.6 is the Freeswitch box and 1.2.3.2 is the Kamailio box. I captured this package on Kamailio box. U 1.2.3.6:5060 -> 1.2.3.2:5060 INVITE sip:2000@test.example.net SIP/2.0. Via: SIP/2.0/UDP 1.2.3.6;rport;branch=z9hG4bKZU1Z76FvDZ26r. Route: <sip:1.2.3.2>;lr. Max-Forwards: 65. From: "908508850000" <sip:908508850000@test.example.net>;tag=KjcgDe0NyKN0B. To: <sip:2000@88.235.112.33:1028>. Call-ID: 72b344f4-1c09-1233-ad8d-0050568e9066. CSeq: 70585075 INVITE. Contact: <sip:mod_sofia@1.2.3.6:5060>. User-Agent: FreeSWITCH-mod_sofia/1.4.14+git~20141119T221113Z~ca1d990cfc~64bit. Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE. Supported: timer, path, replaces. Allow-Events: talk, hold, conference, presence, as-feature-event, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer. Content-Type: application/sdp. Content-Disposition: session. Content-Length: 270. X-AUTH-IP: 159.253.40.138. X-BF-SRC: 2. X-FS-Support: update_display,send_info. Remote-Party-ID: "908508850000" <sip:908508850000@test.example.net . v=0. o=FreeSWITCH 1421813895 1421813896 IN IP4 1.2.3.6. s=FreeSWITCH. c=IN IP4 1.2.3.6. t=0 0. m=audio 28384 RTP/AVP 3 8 0 101 13. a=rtpmap:3 GSM/8000. a=rtpmap:8 PCMA/8000. a=rtpmap:0 PCMU/8000. a=rtpmap:101 telephone-event/8000. a=fmtp:101 0-16. a=ptime:20. I use standart location lookup and relay procedure in Kamailio default script. Is there anything wrong with this INVITE package and how can I trace this issue on Kamailio box? Thank you. /Volkan